Airbus had been studying the idea of a more efficient A320 model for a number of years, but it wasn’t until 2008-2009 with the development of the Pratt & Whitney PW1100G and CFM International LEAP that they could proceed. If I remember correctly, the wing design on the A320neo is quite different to accommodate the larger-diameter PW1100G and LEAP-1A engines. Of course, it required an all-new engine pylon design and a changed landing gear design, too.
You are correct, but the main thing with the 320 retrofit vs the 737 is that the 320 was far enough off the ground to keep the engine under the wing... Yes it needed work.. but it still remained under the wing.
It wasn’t hey, strap that new engine on with no other work, didn’t mean to imply that’s all it was... It was indeed work for the 320 to get those more fuel efficient engines on it.