I'd never use FA as a basis for changing my plans. Any of the "Big 4" airlines will reallocate a plane when necessary. I'm guessing the ULCC would be the only ones who wouldn't, but might in extreme situations.
Last month, I was supposed to fly out of Medford, Oregon first thing in the morning. The plane was supposed to arrive the night before. My flight got cancelled, I think FA still showed the incoming plane was enroute. Got put onto another flight that they ended up sending an empty plane from California to service the flight.
The empty plane didn't show up on FA (as our flight) until less than 30 minutes before it landed.