Why not "Word is God"? Why past tense? Does it mean word was god initially but not anymore?

Even if John had believed the word was created temporally, just as the later Roman church believed in their doctrines of begetting or coming into being of the Son through the Father's body. We could simply reject it as his personal erroneous beliefs, but that is not the case. John is writing about the preexistence of Jesus in temporal pane, he is placing him in the beginning, before Adam (this is in light of the fact that the Jews already believed these kinds of preeminence and divine nature of the Messiah, and described him as Memra/word of God, see Mishnah). John's argument is to show the word's deity as equal as the father's, he doesn't show him to be created as a smaller god. The language of the topic doesn't talk about the beginning of Jesus, but of the world, as in Genesis 1; see footnotes on NET Bible. It presents him as the creator of all things, and as the source of life; nothing indicates he himself had a beginning or was created, as some pagans believed. Again, we should not only keep the whole corpus of the NT in mind, as the author is supposed to share the same beliefs about Jesus as the others (Paul calls him the beginning Col 1:18), but also the corpus of the Jews concerning the divine Messiah. The deity of the Messiah became controversial perhaps only among the pagan or gentile Christians, and consequently among the Jews in retaliation to the Christian message.

Past tense is important since he is describing the Jesus's past, and how he, the word became flesh, in v14. It indeed got changed in time. Expositor's GNT commentary states,

Joh 1:1. In the first verse three things are stated regarding the Logos, the subject ὁ λόγος being repeated for impressiveness. Westcott remarks that these three clauses answer to the three great moments of the Incarnation declared in Joh 1:14. He who was (ἦν) in the beginning, became (ἐγένετο) in time; He who was with God, tabernacled among men; He who was God, became flesh. (1) ἐν ἀρχῇ ἦν ὁ λόγος. ἐν ἀρχῇ is here used relatively to creation, as in Gen 1:1 and Pro 8:23, ἐν ἀρχῇ πρὸ τοῦ τὴν γῆν ποιῆσαι; cf. 1Jn 1:1. Consequently, even in the time of Theophylact it was argued that this clause only asserts that the Logos was older than Adam. But this is to overlook the ἦν. The Logos did not then begin to be, but at that point at which all else began to be He already was. In the beginning, place it where you may, the Word already existed. In other words, the Logos is before time, eternal. Cf. Col 1:18

If you have an objection to his deity due to his change into a man, it is a mere philosophical objection. If he was God, he could not cease to be God in a permanent sense, even if someone believes that, I wouldn't see that as much of an error as the view that turns Jesus into a created or temporal begotten entity.

See the Christology podcast teachings by Dr Craig of reasonablefaith.

This is known as Kenotic Christology. It comes from the Greek word kenosis which means “an emptying.” It is used in Philippians 2:5-7 to characterize Christ’s incarnation. There you will remember Paul says that Christ did not consider equality with God, a thing to be grasped, but he emptied himself taking the form of a servant. Kenotic theology attempted to exploit this idea of Christ’s emptying himself in taking on human nature.[5]

We can define Kenoticism as the view according to which Christ, in the incarnation, ceased to possess certain attributes of deity in order that he could become truly human. He literally gave up some of the divine attributes in order to become a human being. This raises all sorts of questions about the extent of the kenosis – how far did this emptying go? It raises questions about the relationship between the Logos – the second person of the Trinity – and the man Jesus. It also raises questions about the status of the divine attributes as to which could be given up and which could not be surrendered. Kenotic theologians answered these questions in various ways.

Kenoticism represents a non-Chalcedonian approach to Christology. Why? Because it holds that the Logos, in becoming incarnate, changed in his nature where as you will remember the Council of Chalcedon says this is without change. Yet, according to the Kenotic theologians, the Logos did change in becoming incarnate. This raises the question as to whether or not Kenoticism didn’t in fact imply a denial of the deity of the incarnate Christ. If he gave up divine attributes then even if he were the same person after the incarnation, had he thereby ceased to be God? D. M. Baillie, in his book, God Was In Christ, asks,

Does Christianity, then, teach that God changed into a Man? . . . That at a certain point of time, God . . . was transformed into a human being for a period of about thirty years? It is hardly necessary to say that the Christian doctrine of the Incarnation means nothing like that. . . . it would be grotesque to suggest that the Incarnation has anything in common with the metamorphoses of ancient pagan mythology . . .[6]

In these metamorphoses remember Zeus could turn into a swan or he could turn himself into a bull or other sorts of embodied forms. Baillie protests that this would be grotesque to think of the incarnation as being like these metamorphoses in ancient pagan mythology. He says, “the deity and humanity of Christ are not merely successive stages . . . as if He had first been God, then Man, then after the days of His flesh were past, God again, with manhood left behind.” No! The doctrine of the incarnation is the doctrine that Christ was God and man simultaneously. Baillie therefore charges that kenosis, while affirming that the Son of God keeps his personal identity in becoming human, nevertheless he has divested himself of the distinctly divine attributes so that in becoming human he ceased to be divine. If Jesus is in every sense human then the Kenotic theologian is in the position of saying that God has turned himself into a human being which seems absurd.

I think the deeper question raised by Kenotic Christology is the content of the divine nature. That is to say, the question is to which properties are essential to deity, to divinity. Baillie holds that any change in God is an essential change from deity. But it is exactly at this point that the Kenotic theologians question the traditional doctrine. They argue that many of God’s most prominent attributes such as omnipotence, omniscience, omnipresence are merely contingent properties of God, not essential properties. Therefore he could give up these properties and still remain God.

Verses 1 to 5 are intended to define the Word by its relation to:

• time and eternity (in the Beginning means eternal)
• equality with God as a whole (the Word was God)
• God as distinct persons (the Word was with God)

The continuing existence and journey of the Word is the subject of verses 9 to 19. It again uses past tense, but that is because John is describing the incarnation and arrival of Jesus, which is now in the past as John writes.

John is very deliberate about using time references that make careful distinctions. An important example is found in his other book, Revelation.

Revelation 1:4 says "Grace to you and peace from him who is and who was and who is to come." This emphasizes existence now, because "who is" comes first.

Revelation 1:8 is much the same: “I am the Alpha and the Omega,” says the Lord God, “who is and who was and who is to come, the Almighty.” But by prefacing it with Alpha and Omega, the eternal existence of God from beginning to end is emphasized.

Revelation 4:8 changes the order: “Holy, holy, holy, is the Lord God Almighty, who was and is and is to come!” Now God's eternal existence is given in chronological order. This is followed by the elders declaring the worthiness of God because he created all things, thus going back to the beginning, so "who was" is emphasized.

Revelation 11:17 then makes a profound and subtle change:

“We give thanks to you, Lord God Almighty,
    who is and who was,
for you have taken your great power
    and begun to reign."

The phrase "and is to come" has been dropped! That is because time is ending and eternity has been reached by the faithful, for the Lord God Almighty now has no more work to do; the uncontested reign of heaven over all things has begun.

So John has a habit of making careful reference to time, eternity and the enduring nature of God throughout his writings.

Because past tenses in English

Ignoring the Greek, it's the only viable English sentence construction when discussing a past event. If I was the soccer club president in 2020 when we built the new clubhouse, and I am still president, I will say 'I was the president when we built the new clubhouse', not 'I am the president when we built the clubhouse'.

In the beginning (past event), the Word was with God, and the Word was God.

Jesus is still the Word, the Logos, the reason. That's not in question.