My question is fairly obvious: if am entitled to ZL fares on a given airline, but decide to swap to another airline operating the same route as well, am I correct in assuming I will not be able to use my ZL ticket with them if the agreement with that other airline is for ZM?

Is is possible to pay up the difference at the airport, similar like you sometimes have to pay extra taxes if you route otherwise (and more expensive) than first forseen, or is it not possible to pay the fare difference between ZL and ZM at all?

Just curious to your answers/experiences! Thanks.