LiSrt:
While I cannot speak directly to A380, the installation characteristics of the engines are VERY different between OEMs:
1) Configuration of the structural interface to the pylon (so Step One: replace the pylon)
2) Instrumentation and control differences (2 vs 3 rotors, temp/pressure limits,... ) (Replace wiring & connectors, new display software...)
3) Complete a flight test program (OEI, etc) and publish documentation...
In short, it's a BIG engineering job, usually tackled only by (e.g.) GE, P&W or R-R Flight Test Center, or Boeing/Airbus flight test operation.