There certainly would be other costs associated with that, but the primary motivator for not redesigning their aircraft was maintaining type certification, not

There certainly would be other costs associated with that, but the primary motivator for not redesigning their aircraft was maintaining type certification, not the cost of a clean redesign. If the Max8 could keep the same type certification, pilots could cross over without long and expensive training, and without losing their rating on their prior aircraft, making adoption a no brainer. If the Max8 doesn’t keep a type rating, there’s less of an incentive for current 737 customers to buy a Max8, since they’ll have to go through the same logistical hurdles that introducing a A321neo would have. Plus Southwest basically won’t buy anything but 737s, so maintaining that type rating was critical to selling the new plane to one of their biggest customers.