We're using custom-built system utilizing game version and profile-like structure ^-^.
Considering the amount of changes to things affecting the playthrough at this stage of the development, it's unfeasible or in some cases even impossible to make playthrough saves compatible between game versions, so for the time being each version of the game uses dedicated profile for playthrough saves.
Persistent data (gallery) is shared between multiple versions of the game, but if we'd made changes breaking it, we'd need to use different persistent save file name.
This way, no matter which version of the game you're playing, you never lose any progress you made in previous versions you had played.