When and how can a pope revoke the decrees of an ecumenical council?

Faith and Morals/Discipline

Gregory X, to avoid a repetition of the too lengthy vacancies of the papal see, caused it to be decided that the cardinals should not leave the conclave till the pope had been elected. This constitution which inflicted certain material privations on the cardinals if the election was too long delayed, was suspended in 1276 by Adrian V, and a few months later revoked by John XXI

This amounts to a disciplinary matter being decreed, and then revoked. Whereas matters of faith and morals—the Church teaching matters of the Faith, which may, under certain circumstances, enjoy infallibility ¹—are by nature irreformable, as they pertain to that which is a truth of the Faith (e.g. the divinity of the Holy Ghost).

The Two Kinds of Decrees by a Council

The Catholic Encyclopedia, in its article for Infalliblity, references the Council held by the Apostles and bishops in Jerusalem (recorded in Acts 15). There were two aspects to the 'decrees' that came out of this Council. Though it is pre-ecumenical, it has, like other Councils, the following basic 'format':

Issue/Heresy at Hand

15:1 "Some men came down from Judea and began teaching the brethren, "Unless you are circumcised according to the custom of Moses, you cannot be saved."

Meeting of Bishops

15:6 "The apostles and the elders came together to look into this matter."

Pope Settles Matter, Ultimately

15:8 "After there had been much debate, Peter stood up and said to them..." 15:12 "All the people kept silent..."

Council Document(s) Crystallizing the Outcome

15:23-29

Council Ratified

15:28 "For it seemed good to the Holy Spirit and to us to lay upon you no greater burden than these"

In this Council, as in most, there are the disciplinary matters (or, regulations):

Acts 15:28-29

For it hath seemed good to the Holy Ghost and to us, to lay no further burden upon you than these necessary things: That you abstain from things sacrificed to idols, and from blood, and from things strangled, and from fornication; from which things keeping yourselves, you shall do well. Fare ye well.

And the matters of faith and morals (of prime importance—here, how one is reconciled to God by faith; with or without circumcision):

Acts 15:7-12 And when there had been much disputing, Peter, rising up, said to them: Men, brethren, you know, that in former days God made choice among us, that by my mouth the Gentiles should hear the word of the gospel, and believe. And God, who knoweth the hearts, gave testimony, giving unto them the Holy Ghost, as well as to us; And put no difference between us and them, purifying their hearts by faith. Now therefore, why tempt you God to put a yoke upon the necks of the disciples, which neither our fathers nor we have been able to bear? But by the grace of the Lord Jesus Christ, we believe to be saved, in like manner as they also. And all the multitude held their peace...

Of course, since the Holy Ghost approved of their disciplinary decision, and thus is was infallible in that sense, and in this special, explicitly revealed case, it's not necessarily the case that a matter of discipline is the best, is ideal etc. Only matters of faith and morals, as the Pope teaching Christ's flock (cf. Lk 22:32; Jn 21:15-17), can enjoy infallibility (and therefore irreformable) under the respective circumstances.

A Real-Life Example

A real-life example from a real Ecumenical Council, take the first of them all, Nicaea I, canon 20:

Forasmuch as there are certain persons who kneel on the Lord's Day and in the days of Pentecost, therefore, to the intent that all things may be uniformly observed everywhere (in every parish), it seems good to the holy Synod that prayer be made to God standing.

Or canon 4:

It is by all means proper that a bishop should be appointed by all the bishops in the province; but should this be difficult, either on account of urgent necessity or because of distance, three at least should meet together, and the suffrages of the absent [bishops] also being given and communicated in writing, then the ordination should take place. But in every province the ratification of what is done should be left to the Metropolitan.

These are matters of discipline, and do not concern the Faith directly.

This (Creed/Ecthesis of the Synod at Nice):

...And whosoever shall say that there was a time when the Son of God was not (ἤν ποτε ὅτε οὐκ ἦν), or that before he was begotten he was not, or that he was made of things that were not, or that he is of a different substance or essence [from the Father] or that he is a creature, or subject to change or conversion — all that so say, the Catholic and Apostolic Church anathematizes them.

Are matters of faith and morals.

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You said,

In another article, the CE says that John XXI's action was done "with the consent of the cardinals," but it's not clear to me if such consent was strictly necessary.

Such a consent is not necessary for the kind of infallible statement issued by a Pope 'ex cathedra,'² nor to revoke what is but a disciplinary matter (and therefore reformable by nature).

Pope Pius IX, Vatican I, Session 4, cap. 4, 9:

Therefore, faithfully adhering to the tradition received from the beginning of the Christian faith, to the glory of God our Savior, for the exaltation of the Catholic religion and for the salvation of the Christian people, with the approval of the Sacred Council, we teach and define as a divinely revealed dogma that when the Roman Pontiff speaks ex cathedra, that is, when, in the exercise of his office as shepherd and teacher of all Christians, in virtue of his supreme apostolic authority, he defines a doctrine concerning faith or morals to be held by the whole Church, he possesses, by the divine assistance promised to him in blessed Peter, that infallibility which the divine Redeemer willed his Church to enjoy in defining doctrine concerning faith or morals. Therefore, such definitions of the Roman Pontiff are of themselves, and not by the consent of the Church, irreformable.

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Under what conditions is it acceptable for a pope to revoke an ecumenical council's decrees?

Under any, really: as long as said decrees are concerned with disciplinary matters (not intrinsically defined based on or dependant purely upon some dogma or matter of faith) which are by nature always revokable.

Possible Controversy

A controversial area is where discipline and faith and morals meet or are so connected as to make it unsure just what can be reformed. An example is when Pope Pius V, in a document called Quo Primum, forbade the changing of the Roman Rite of the Mass henceforth and forever, by anyone whoever:

...for its most becoming that there be in the Church only one appropriate manner of reciting the Psalms and only one rite for the celebration of Mass . . . Let all everywhere adopt and observe what has been handed down by the Holy Roman Church, the Mother and Teacher of the other churches, and let Masses not be sung or read according to any other formula than that of this Missal published by Us. This ordinance applies henceforth, now, and forever, throughout all the provinces of the Christian world, to all patriarchs, cathedral churches, collegiate and parish churches, be they secular or religious, both of men and of women – even of military orders – and of churches or chapels without a specific congregation in which conventual Masses are sung aloud in choir or read privately in accord with the rites and customs of the Roman Church. This Missal is to be used by all churches, even by those which in their authorization are made exempt, whether by Apostolic indult, custom, or privilege, or even if by oath or official confirmation of the Holy See, or have their rights and faculties guaranteed to them by any other manner whatsoever.

This new rite alone is to be used unless approval of the practice of saying Mass differently was given at the very time of the institution and confirmation of the church by Apostolic See at least 200 years ago, or unless there has prevailed a custom of a similar kind which has been continuously followed for a period of not less than 200 years, in which most cases We in no wise rescind their above-mentioned prerogative or custom . . .

We specifically command each and every patriarch, administrator, and all other persons or whatever ecclesiastical dignity they may be, be they even cardinals of the Holy Roman Church, or possessed of any other rank or pre-eminence, and We order them in virtue of holy obedience to chant or to read the Mass according to the rite and manner and norm herewith laid down by Us and, hereafter, to discontinue and completely discard all other rubrics and rites of other missals, however ancient, which they have customarily followed; and they must not in celebrating Mass presume to introduce any ceremonies or recite any prayers other than those contained in this Missal

He considered it important enough a decision that he said:

...no one whosoever is permitted to alter this...statute, ordinance, command, precept, grant, indult, declaration, will, decree, and prohibition. Would anyone, however, presume to commit such an act, he should know that he will incur the wrath of Almighty God and of the Blessed Apostles Peter and Paul.

Or the very much analagous canon of the Council of Trent (Session 7, canon 13):

"If anyone says that the received and approved rites customarily used in the Catholic Church for the solemn administration of the Sacraments can be changed into other new rites by any pastor in the Church whosoever, let him be anathema."

(Let him be anathema means outside of the Church, excommunicated ipso facto— heretic, not a member of the Church, hence its original meaning—'under God's curse'—applies only to members of the Church who would presume to commit such a heresy etc).

The controversy being that Pope Paul VI did precisely this, in changing the Roman Rite of the Mass. Since it is technically a matter of discipline, though, this amounts to disobedience to Pope Pius V's solemn decree and wish or ("Our...statute, ordinance, command, precept, grant, indult, declaration, will, decree, and prohibition")—("It is Our will, therefore, and by the same authority, We decree that, after We publish this constitution and the edition of the Missal, the priests of the Roman Curia are, after thirty days, obliged to chant or read the Mass according to it")—and not outright heresy. So goes one side of the controversy among Traditional Catholics:) It could be argued that Pope St. Pius V simply overreached with his power to bind to a disciplinary matter and so it has no more effect than an otherwise mundane disciplinary wish or statute. It could also be argued that there was no sufficient reason to alter said Rite.

However, as in the case of your question and that decree, the disciplinary matter of making sure a Pope is elected quickly to avoid a prolonged period of sede vacante (no Pope) (your question), this is not in question at all. It is clearly a matter of discipline.

Judgement over any confusion (or interpretation of Church teaching in general), however, is left in the hands of the Pope and the Church, of course, not to the individual.

As the Catholic Encyclopedia mentions in the abovementioned article on infalliblity:

...the Church is infallible in her objective definitive teaching regarding faith and morals, not that believers are infallible in their subjective interpretation of her teaching. This is obvious in the case of individuals, any one of whom may err in his understanding of the Church's teaching; nor is the general or even unanimous consent of the faithful in believing a distinct and independent organ of infallibility.

1 ^{Catholic Encyclopedia, Infallibility | Wikipedia, Papal Infallibility (from the teaching of Pius IX, Vatican I, Pastor Aeternus)}

2 ^{'ex cathedra,' that is, 'from the chair' [of Peter]—not literally; refers to a specific kind of authoritative statement or decree, or excercising of a certain office as Pope; cf. Matthew 23:1-3}