Infallibility doesn’t have anything to do with changing anything, but defining things on matters of Faith and Morals. Both the Church in Council and the Pope when speaking Ex-Cathedra have the promise of Infallibility.
This is an ultramontanist myth.
There had never been an application of this apparent authority until the 19th century.
No bishop, no churchman of any sort, declared Christian dogma upon his authority alone except in those instances where heretics were teaching new things. The bishops of Rome only began to do so in 1854AD, without the benefit of prior approval by a church council of bishops.
For example, when the Church defined “Trinity” to explain clearly the Christian belief of God - that there is only One God and that He is 3 Persons: Father, Son and Holy Spirit. It was something that had been handed down for centuries, but some confusion arose in the matter - specifically within the mind and teachings of Bishop Arius who began to teach that Jesus was not God. The Pope called a Council and the Bishops got together and with the direction of the Holy Spirit, they made an Infallible statement about God. They did not change anything, but simply infallibily defined the teaching of the Church.
No Pope called this council, you are mistaken.
At best (if it were true) this argument would help bolster the concept of Papal Universal Jurisdiction, not Papal Infallibility (the actual decisions were still the work of a council). Considering that the Pope did not actually call councils at the time does not lend credence to even this much authority.
No Bishop of Roma called any of the seven councils, the one’s pertaining to this matter (Nicea and Constantinople) were called by the emperors Constantine and Theodosios respectively (the bishops of Roma had no say in the matter). The emperors interest was purely political and self-serving, they desired a united Christian faith to help hold the diverse people together as one nation.
The resulting Councils are a good demonstration of the conciliar nature of the church. The infallibility of the church… the mind of the church… is expressed through the entire church, and this ideally seems best revealed by a council of the bishops together, across synodal lines meeting simultaneously (which is why the Latin church copied the model for itself later). All that remains is for the local synods to subsequently affirm the decision of the gathered bishops, and agree to teach accordingly.
Conciliarity is the earlier form of church governance, and still operative in the Orthodox east. Even more telling, the non-Calcedonians (on their own since 451AD) are also conciliar, it is what they knew and inherited from the church before the sad dispute which separated them from your church and mine. Conciliarity in the west died with the Council of Basil, and now is considered a type of heresy in the western church.
The ultramontanist Papacy is a new (post-schism, post council of Constance) thing.
BTW, I believe Arius was a priest when he proclaimed his own theology. I don’t know if he ever became a bishop.
The Pope has the same authority and has exercised it on a couple of occasions.
As stated above, this is a 19th century novelty.
Interestingly, it has only been invoked twice (the first time [1854AD] before the Pope was even authorized by his church to do so) and both times not on Christological matters, but Mariological ones, which seems to be some sort of abuse of the justification for such a power.