Compared to getting the MAX back into the air, changing seats and repainting are minor details. Having differing seating capacities isn’t insurmountable, especially if the lower capacity aircraft are based at a major hub and assigned to appropriate flights, other airlines cope.
Depending on the price, anti dumping laws might be a problem if the aircraft are sold below production cost, which may happen as Boeing might have to aim to minimise their losses instead of making a profit.