She says "you can't change the past".

That means that no matter how much timetravelling you do, you won't change anything because your timetravelling was a natural part of time.
We can extend this to see that the future can't change either.
You see, someone from the future, might travel to a section of his own past, which would at the same time be part of someone elses, say Squalls future. Like this:

Squall---------------Where Mr. X returns to.--------Mr X.
Time---->

This is exactly what Ultimecia does. She arrives in our time from the future, and since she can't change the past either, Squall and co can't change their future.

Hence, time doesn't ever change.