Welcome!
I have both the FM2 and the FM3a, both are great cameras and I see no strong reason to change the 2 for a 3a. It's true that the 3a has an aperture priority auto mode (so does the much cheaper FG or the great FE), but really I never use that mode, so both cameras are pretty much the same for me. Keep the FM2 and get an FG (with a useful Program mode) for an 8th of the 3a price.