Engines are separate from the airframe; on big commercial aircraft the engines get swapped/moved around all the time (the engines may well be on lease, separate

Engines are separate from the airframe; on big commercial aircraft the engines get swapped/moved around all the time (the engines may well be on lease, separate to the lease on the airframe). Thus, in this sort of situation it's the engine's history that'll be important, rather than the airframes.