Firenze, Trelawney and predictions

[Christy]

No: HPFGUIDX 81032

I have a question, that has been nagging me for awhile now, so I'm
hoping that somebody here has a thought.

We know that Sybil predicted the prophesy and Dumbledore tells Harry
that at the time it was predicted it could have applied to either
Harry or Neville (and is still up for debate as to which one it may
currently apply to).

We also have head a couple times from Firenze who says that even the
Centaurs have made mistakes reading the stars.

My question is this: Is it possible that the future is set but it can
also be changed by the actions that people take to the point of
completely changing what would have been(like in the case of the
prediction... one rash act by Voldemort and the entire future is
altered)? Which might help explain further why time-turners have been
banned... What do others think? Am I nuts?

Christy