This documentation is always evolving.
Right now projects created in the previous version of Mapcreator will not be available in Mapcreator Next and vice versa. After further developments, the old version of Mapcreator will eventually be replaced by The Next Mapcreator. At this point all previous projects will be available there, regardless of where these were created.
As developments are ongoing, the aim is for the old version of Mapcreator to retire mid 2025.