Easier said than done.
If you change engines, the pylon needs to be changed as well, as each is designed with a specific engine in mind.
The only 'really easy' (more or less) engine change was on older straight pipe powered 707-300's (JT4A engines) that were re-engined with JT3D fans.
Engine/pylon change, rudder Q feel etc, and not all that much else.
This produced an aircraft that was an exact copy of an already certificated aircraft.
On a newer type, the cost may well be prohibitive, not so much the engines as the pylons, engineering etc.