There are times in Mass when the priest can ad lib and “pray from the heart” - the homily is an example. There are other times, however, such as when he is turning the bread and wine into the body and blood of Jesus, that there is no room for ad libbing. The priest must use the words of Jesus - “This is My Body” and “This is My Blood”.
Not all Christian denomination recognize the Biblical roots of the Eucharist and Jesus being present in the appearance of bread and wine. Those Christian denominations should refer to John 6 and they should refer to Matthew 16:18-20 to understand that Christ founded the Catholic Church.
Is that what he changed?
The lack of understanding could be on the other side. In no place do I see Jesus rebuke anybody for not repeating "magic words". Jesus looked upon the heart. Jesus knew what was inside of them. The Catholic Church does not know where the True Church is or Who the Head of it is. (Hint: It ain't in Rome and it ain't the Pope). Jesus fought against religious authorities who ritualized and commercialized the worship of God. They were the ones who incited the people against Him.
Peace be unto you.
Christ is the Rock that the church is built on, not Peter.