What does St. Thomas Aquinas mean by 'alteration' of heavenly bodies in Summa Theologiae?

Alteration (alteratio) is a philosophical term for a type of change/motion (motus or mutatio).

St. Thomas Aquinas defines alteratio in Physica V l. 4 [679.]:

motion/change in quality is called aleration.
_{motus qui est in qualitate, vocatur alteratio}

Thus, a green apple turning to a red apple would be an alteration in the quality of the apple.

Other types of change are:

1. local motion (change in position/location)
2. augmentation or diminution (changes in quantity)
3. substantial (e.g., a frog becoming a prince) vs. accidental (e.g., a green apple becoming a red apple)

cf. §18 (pp. 49-52) of William A. Wallace, O.P.'s The Elements of Philosophy: A Compendium for Philosophers and Theologians.

This brightness will be in these bodies even as it is in the heavenly bodies, in which it causes no heat, because these bodies will then be unalterable, as the heavenly bodies are now.
_{claritas illa erit in istis corporalibus, sicut est in corporibus caelestibus, in quibus caliditatem non causat; quia corpora ista tunc erunt inalterabilia, sicut modo caelestia.}

relies on knowledge of St. Thomas's astronomy, which is basically that of Aristotle, who considered astronomical bodies to be more perfect than terrestrial bodies, not undergoing substantial change; cf.

• pp. 58 ff. "Différence entre la matière des corps célestes et celle des corps inférieurs" of Thomas Litt, O.C.S.O.'s Les corps célestes dans l'univers de saint Thomas d'Aquin.

The closest Litt comes to discussing your passage is where he quotes, on p. 247, Summa Theologica suppl. q. 91 a. 3 "Whether the brightness of the heavenly bodies will be increased at this renewal?" arg./ad 2 (=Super Sent. lib. 4 d. 48 q. 2 a. 3 arg. 2/ad 2). St. Thomas answers the objection that

just as the heavenly bodies are the cause of generation in this lower world by their movement, so are they by their light. But, when generation ceases, movement will cease as stated above (Article [2]). Therefore in like manner the light of the heavenly bodies will cease rather than increase.
_{Praeterea, si innovato homine corpora caelestia innoventur, oportet quod eo deteriorato fiant deteriora. Sed hoc non videtur probabile, cum illa corpora sint invariabilia quantum ad id quod est in substantia eorum. Ergo nec innovato homine innovabuntur.}

by saying that

Movement does not denote perfection in the thing moved, considered in itself, since movement is the act of that which is imperfect: although it may pertain to the perfection of a body in so far as the latter is the cause of something. But light belongs to the perfection of a lightsome body, even considered in its substance: and consequently after the heavenly body has ceased to be the cause of generation, its brightness will remain, while its movement will cease.
_{Ad secundum dicendum, quod motus non importat aliquam perfectionem in eo quod movetur, secundum quod in se consideratur, cum sit actus imperfecti; quamvis possit pertinere ad perfectionem corporis, inquantum est causa alicujus. Sed lux pertinet ad perfectionem corporis lucentis, etiam in substantia sua considerati; et ideo postquam corpus caeleste desinet esse causa generationis, non remanebit mortuum, sed remanebit claritas ejus.}

What does St. Thomas Aquinas mean by 'alteration' of heavenly bodies in Summa Theologiae?

What St. Thomas is saying here is that the heavenly bodies are undergoing changes as to the fact that the entire universe is alive, growing and subject to change, just as the earth and it’s inhabitants are.

For example a sun is just a sun. But after a certain amount of time has past by it will eventually die, just like people do.

When God renews the heavens and the earth at the end of time, all this will change and any alterations will cease all together. Everything will exist in perfect harmony with it’s Creator.

The sun gives energy to life on Earth, and without this star, we wouldn't be here. But like most things in space, even stars have limited lifetimes, and someday our sun will die.

You don't need to worry about this solar death anytime soon, though. Inside the sun, a churning fusion engine fuels the star, and it still has a lot of fuel left — about 5 billion years' worth.

Stars like our sun form when a huge cloud of gas (mostly hydrogen and helium) grows so large that it collapses under its own weight. The pressure is so high in the center of that collapsing mass of gas that the heat reaches unimaginable levels, with temperatures so hot that hydrogen atoms lose their electrons. Those naked hydrogen atoms then fuse together into helium atoms, and that reaction releases enough energy to counter the intense pressure of gravity collapsing the cloud of gas. The battle between gravity and the energy from fusion reactions fuels our sun and billions of other stars in our galaxy and beyond.

What will happen when the sun dies?

But in about 5 billion years, the sun will run out of hydrogen. Our star is currently in the most stable phase of its life cycle and has been since the birth of our solar system, about 4.5 billion years ago. Once all the hydrogen gets used up, the sun will grow out of this stable phase. With no hydrogen left to fuse in the core, a shell of fusion hydrogen will form around the helium-filled core, astrophysicist Jillian Scudder wrote in an article for The Conversation. Gravitational forces will take over, compressing the core and allowing the rest of the sun to expand. Our star will grow to be larger than we can imagine — so large that it'll envelope the inner planets, including Earth. That's when the sun will become a red giant.

For about a billion years, the sun will burn as a red giant. Then, the hydrogen in that outer core will deplete, leaving an abundance of helium. That element will then fuse into heavier elements, like oxygen and carbon, in reactions that don't emit as much energy. Once all the helium disappears, the forces of gravity will take over, and the sun will shrink into a white dwarf. All the outer material will dissipate, leaving behind a planetary nebula.

"When a star dies, it ejects a mass of gas and dust — known as its envelope — into space. The envelope can be as much as half the star's mass. This reveals the star's core, which by this point in the star's life is running out of fuel, eventually turning off and before finally dying," said astronomer Albert Zijlstra, of the University of Manchester in the United Kingdom, in a statement. - When Will the Sun Die?

Thus we are aware of movement, transforms and alterations going on in space all the time. Meteors hit the earth on a daily bases.

Whether the world will be renewed?

We believe all corporeal things to have been made for man's sake, wherefore all things are stated to be subject to him [Psalm 8:5, seqq.]. Now they serve man in two ways, first, as sustenance to his bodily life, secondly, as helping him to know God, inasmuch as man sees the invisible things of God by the things that are made (Romans 1:20). Accordingly glorified man will nowise need creatures to render him the first of these services, since his body will be altogether incorruptible, the Divine power effecting this through the soul which it will glorify immediately. Again man will not need the second service as to intellective knowledge, since by that knowledge he will see God immediately in His essence. The carnal eye, however, will be unable to attain to this vision of the Essence; wherefore that it may be fittingly comforted in the vision of God, it will see the Godhead in Its corporeal effects, wherein manifest proofs of the Divine majesty will appear, especially in Christ's flesh, and secondarily in the bodies of the blessed, and afterwards in all other bodies. Hence those bodies also will need to receive a greater inflow from the Divine goodness than now, not indeed so as to change their species, but so as to add a certain perfection of glory: and such will be the renewal of the world. Wherefore at the one same time, the world will be renewed, and man will be glorified.

Whether the movement of the heavenly bodies will cease?

There are three opinions touching this question. The first is of the philosophers who assert that the movement of the heaven will last for ever. But this is not in keeping with our faith, which holds that the elect are in a certain number preordained by God, so that the begetting of men will not last for ever, and for the same reason, neither will other things that are directed to the begetting of men, such as the movement of the heaven and the variations of the elements. Others say that the movement of the heaven will cease naturally. But this again is false, since every body that is moved naturally has a place wherein it rests naturally, whereto it is moved naturally, and whence it is not moved except by violence. Now no such place can be assigned to the heavenly body, since it is not more natural to the sun to move towards a point in the east than to move away from it, wherefore either its movement would not be altogether natural, or its movement would not naturally terminate in rest. Hence we must agree with others who say that the movement of the heaven will cease at this renewal of the world, not indeed by any natural cause, but as a result of the will of God. For the body in question, like other bodies, was made to serve man in the two ways above mentioned (Article 1): and hereafter in the state of glory man will no longer need one of these services, that namely in respect of which the heavenly bodies serve man for the sustenance of his bodily life. Now in this way the heavenly bodies serve man by their movement, in so far as by the heavenly movement the human race is multiplied, plants and animals needful for man's use generated, and the temperature of the atmosphere rendered conducive to health. Therefore the movement of the heavenly body will cease as soon as man is glorified.

Whether the brightness of the heavenly bodies will be increased at this renewal?

The renewal of the world is directed to the end that, after this renewal has taken place, God may become visible to man by signs so manifest as to be perceived as it were by his senses. Now creatures lead to the knowledge of God chiefly by their comeliness and beauty, which show forth the wisdom of their Maker and Governor; wherefore it is written (Wisdom 13:5): "By the greatness of the beauty and of the creature, the Creator of them may be seen, so as to be known thereby." And the beauty of the heavenly bodies consists chiefly in light; wherefore it is written (Sirach 43:10): "The glory of the stars is the beauty of heaven, the Lord enlighteneth the world on high." Hence the heavenly bodies will be bettered, especially as regards their brightness. But to what degree and in what way this betterment will take place is known to Him alone Who will bring it about.

Whether the elements will be renewed by an addition of brightness?

Just as there is a certain order between the heavenly spirits and the earthly or human spirits, so is there an order between heavenly bodies and earthly bodies. Since then the corporeal creature was made for the sake of the spiritual and is ruled thereby, it follows that corporeal things are dealt with similarly to spiritual things. Now in this final consummation of things the lower spirits will receive the properties of the higher spirits, because men will be as the angels in heaven (Matthew 22:30): and this will be accomplished by conferring the highest degree of perfection on that in which the human spirit agrees with the angelic. Wherefore, in like manner, since the lower bodies do not agree with the heavenly bodies except in the nature of light and transparency (De Anima ii), it follows that the lower bodies are to be perfected chiefly as regards brightness. Hence all the elements will be clothed with a certain brightness, not equally, however, but according to their mode: for it is said that the earth on its outward surface will be as transparent as glass, water as crystal, the air as heaven, fire as the lights of heaven. - Question 91. The quality of the world after the judgment