You can do it, but it's not cheap for the airlines to retrain pilots. Adding another type of aircraft to an existing fleet is also expensive, because it's difficult to maintain currency on multiple aircraft types. This means that you can't swap crews between flights and you need to maintain more reserve crews. This is one of the reasons Southwest only flies 737s -- they never need to retrain for another aircraft type, they can swap any crew for any other crew, and they only need reserves for one aircraft type.

The inherent flight characteristics of the 737MAX are very similar to the 737NG. The difference is only in abnormal, high AoA maneuvers, when the aircraft is being mishandled. MCAS was supposed to correct for this situation. Unfortunately the final MCAS design was highly failure prone and instead of protecting from an abnormal situation, it caused the plane to become uncontrollable.