May an airline replace an airliner's engines without the aircraft manufacturer's involvement? Around a year ago in our country, there was an advertisement on TV regarding an airliner's program to modernize their airplanes' power plants. This airline has various types of Airbus and Boeing aircraft.

I am not sure whether all the airplanes are using the same power plant brand or not. The advertisement shows that one power plant's manufacturer is GE. So I am not sure what is the meaning of "modernization" in their advertisement regarding the power plant. But instinctively, that should be to replace it with a newer, better version.

So my question is, does the airplane manufacturer allow the airline to replace their own power plants without the airplane 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.