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?