What are the obstacles to buying an aircraft with one manufacturers engines and replacing said engines with another manufacturers engines?
Qantas took a handful of B767's off of BA in the 90's. These were RRs. The rest of the QF 767's were GE and they never changed the engines to make them common, so not sure if they can do it that easily.
There are sometimes other configuration issues associated with the types of engines making them non-interchangable.