I’ve upgraded my MK3 to S and then S+ and my MMU2 to S and then 3. Starting from the launch model of the MK3 and ending with MK3S+ and MMU3 costed me significantly less than buying a new printer. Yes, the upgrade kits across models are not worth it (MK3 to MK4 or MK4 to Core one), but what do you expect? At least they offer the option, and details documentation, you can even source all the parts by yourself.