Except apparently, per the OP, this does happen, as the FAs working the flight weren't supposed to originally. Who knows when the flight would have left, if at all, if they didn't pull that crew.

Certainly more likely for an aircraft/crew to get swapped at a hub. But even at a non-hub, it can happen - the only specific reason I can think of where it would be impossible, aside from crew rest/work restrictions, is if the (regional) operating carrier/crew is different.