Are you planning to store a LOT of programs or just very big ones? Either way, I would assume it's easy enough to just store any excess offline and load them as needed. Have you posted any code for that control? Does it just use G code? I was surprised at how little room code takes up.
Anyway, I know nothing about that control or how to upgrade the memory. It sounds like a fantastic machine with a very obscure control. If it works, I'd just run it as is (and be thankful that it even works). It seems that many folks in the Tree forum are desperately trying to resurrect machines that are slowly giving up the ghost.