1) thanks for the reply
thats what I wanted to try to do first - but then decided to wait a bit first
2) copied the files like in your video but itch wanted to still update the game - so to avoid this mess and further trouble with the itch-updater - I backuped the saves and deleted the game to re-install it
3) deletion worked but re-install doesn't work either
3a) for the HiDrive folder - "An error occured during installation No manager for installer unknown"
3b) for the MEGA folder - "An error occured during installation runtime error: slice bounds out of range"