This works the same was as the Pro 3, in which the User sounds can be copied over to the Factory locations (using the procedure described by Hamudi2000), so that all 400 locations are available for your own sounds.
I like this model of storage, because it "protects" part of the memory from being accidentally overwritten, while still letting you to a full-scale factory sound replacement.
I make a LOT of Prophet 5 sounds, and I'm probably eventually going to have to curate and move the memory around. My plan is to at least keep the "original" Prophet patches, for historical reasons, even though they're dated.