"Can an aircraft originally fitted with one engine type have a different type fitted later?"
In short, no, not for a modern airliner. Apart from purchasing the new engine type (and parts, repair contracts etc), there would need to be extensive (read, basically complete changeout) of engine mounting systems, instrumentation/avionics and a lot of plumbing.
Basically uneconomical.
The 787, however, with it's all-electric engines, was originally touted as being able to have swappable engine types (mostly at the request of leasing customers, who didn't want to be stuck with a fleet of unpopular aircraft because of a punt on an engine type that turned out to be poor). I'm not sure if that (swapping) is still the case.