While swapping engine types is not "impossible", it is cost prohibitive - aside from nearly everything under the wing being different, much of the interface wiring is different (as just one example, Rolls has a heated inlet probe - roughly 500 watts aircraft power with the associated aircraft wiring, circuit breakers, etc., while GE's inlet probe in unheated).
As you may know, Boeing put two versions of the CF6-80C2 engine on the 767 - "PMC" and "FADEC" (PMC being a hydromechanically controlled engine with a 'supervisory' electronic control). Occasionally an operator has come in and asked to convert a CF6-80C2 767 from PMC to FADEC. Once they heard the cost (several million dollars) they quickly lost interest. Swapping between engine manufactures would be even worse.
Initially Boeing attempted to make the 787 engines 'plug and play' but I don't think it worked out...