Before we can answer the question about changes to the New Testament, we have to define what the early Christian church actually looked like.
Modern scholars have been able to acknowledge that the early church was not one single group of Christians whom all believed the same thing. Variations in doctrine and different Christian groups started forming as early as the mid 1st century. By the early 2nd century the Christian church consisted of Orthodox believers, Marcionites, Ebionites and Gnostics who all believed in Jesus but chose to interpret his message and his purpose differently. All these groups chose different books to be included in their collection of scripture and we have plenty of evidence to suggest that all these groups made changes to the books that we still have in our Bibles today. We know they did this because we have copies from different regions that still contain these changes today, as well the comments made by some of the earliest Church fathers.
By the late 2nd century Dionysius, the Bishop of Corinth actually complained that even the Gospels were being changed by copiers at this stage. He stated the following:
"As the brethren desired me to write epistles (letters), I did so, and these the apostles of the devil have filled with tares (changes), exchanging some things and adding others, for whom there is a woe reserved. It is not therefore, a matter of wonder if some have also attempted to adulterate the sacred writings of the Lord, since they have attempted the same in other works that are not to be compared with these."
The earliest complete copies that we have today come from the 4th century BCE and it's near impossible to know for certain how much these 4th century copies differ from these corrupted 2nd century copies.