No one is changing anyone's words.
Christ was speaking prophecy and used 'this' to refer to the group He was speaking about, the generation that would have to flee when the anti-Christ came into the Temple.
It wasn't Christians He was speaking about since Christians would not be concerned about fleeing on the Sabbath!
Christ was speaking to those who believed in Him. That’s the definition of “Christian.”
When reading the Bible, it’s important to keep in mind whom Jesus was speaking to and whom the letters (such as Revelation, which was a letter) were addressed to. For example, it would be a bit misleading on John’s part to write that God “will keep ‘you’ from the hour” if he didn’t really mean the people he was writing to.
If you were a persecuted Christian and received a letter containing words of comfort such as the above, would you really think that comfort was meant for someone else in another century?