May an airline replace an airliner's engines without the aircraft manufacturer's involvement?
If the airline owns their airplanes outright, then they don't need permission per-se from the manufacturer, but the new engine would have to be approved by the manufacturer for that type of plane. So the airline couldn't just pick some type of jet engine &amp; decide that they're going to figure out how to mate it to their jets; if that combination hasn't already been tested &amp; proven by the manufacturer, it would void the manufacturer's certifications and render the aircraft un-airworthy. So nobody goes that route.

On the other hand, if the airline leases their jets, they can only do what the owner approves.