When the Max comes online it will be a different code (e.g. 73M?, 739?). I don't think we'll see a different code for the 738 as it is configured now vs. 738 with the future seating config. You could be assigned a current variant at time of booking and then get changed to a new config as departure gets closer.
the max is 7M8.
also the Max 8 is already flying.