It's always possible that it could change, even after departure time and pushback from the gate. I had a -700 aircraft last week that picked taxi-out to have a mechanical issue, and once back at the gate, the only available aircraft we had to swap with was a -300, so we did.

That said, most of the time (90%+ if I had to guess) it'll be what it's scheduled for, but in a biz that's as dynamic as the airline biz, changes cannot be totally ruled out.