Unlikely an airline that doesn’t own or lease a max will swap one in.
Sadly the majority of people will choose the cheapest result and voting with your wallet doesn’t work. Which is why we need regulation and can’t leave things to the free market, despite what the libertarians say.
Perfect use for VC money: Reimburse customers when the airline switches planes or flights in perfectly legal manner and customers end up with a Boeing plane anyways.
And also, airlines can switch planes at any time, you won't find out for sure what plane you are taking until you sit on it.