The only easy way around it is to just cancel the flight when an aircraft with a premium cabin is not available to meet the planned inventory sold on that flight. Legacy airlines typically have not had to deal with hybrid cabin configs where there hasn't been a premium cabin from new in a specific aircraft, they may just have to deal with a seat count difference and move a few PAX if necessary with a tail change.

I can't see the current situation getting much better until the cabin transformation has been completed, which is not being helped by the slow delivery from Boeing, making it tricky to plan for aircraft availability for the 2 transformation lines and also plan what tail configs are going to be available 6 months or so in advance to make the right inventory available.