To add to the other comment. The airframe was different enough were it would have to be listed as a different type of plane. This means that any airline looking to buy the 737 max would have to train all of their pilots on this frame, which is comes at a huge cost and would reduce airlines incentive to buy it. So what did Boeing do? Somehow they managed to convince regulators that the 737 Max is no different than the 757, so pilots would not need new training, they just needed to watch a few videos on the new platform. However, this was a complete lie, the 737 Max had engines that were bigger and were sitting further back in the fuselage, this gave the 737 Max a tendency to pitch upward. To counteract this, and to help Boeing sell their lie, the program their software to counteract this issue (the MCAS, which is not the autopilot). So this software gave the illusion that the 737 flies like the 757 (hence why pilots wouldn’t need new training). However, Boeing installed only a single sensor to detect if the plane pitched upwards, in two cases the sensors failed and told the plane that it was pitching upwards when in fact it was flying straight. This caused the plane to go into a nosedive, and because MCAS was not autopilot the pilots had no idea what was happening, let alone how to turn it off. Which resulted in two airplane crashes in the span of a few months that killed everyone on board.