I have a 2003 Subaru Forester x with the 2.0l engine. How hard would it be to put a 2.5l engine in.

Not a straight swap, the ecu will be different ans also I feel that the loom will also need swapping, not sure about exhaust and engine controls, I would contact a wrecker specialising in these cars as they will be doing component swaps every day and will know for sure about the finer points needing attention.