The airlines can and do switch planes at the last minute, though. And I imagine part of the selling point of the MAX not requiring any recertification is that it meant they should be able to also swap a MAX for a non-MAX 737 like this.

Certainly there have been reports of pilots complaining that they didn't find out they'd be flying a MAX 8 until the last minute, and having to get "trained" on it during their ride to the airport.