Hi Eggy,
Migrating files between workspaces can only be done through shared drives. Both the old user and the new user must be managers of the drive.
So essentially for each user you would:
- Create a dedicated shared drive and add both the source and target users to it.
- Move all the source users My Drive files to the shared drive (doing it "as" the source user via GAM)
- As the target user move all those files and folder from the shared drive to their My Drive.
- Delete the shared transfer drive.
You could also I suppose use a single shared drive and have folders for each users drive, it just depends on how private you need to need to keep everything during the migration. The steps above can all be automated with a script using GAM, potentially running for multiple users in parallel, which is another reason I think it's cleaner to have a dedicated transfer drive per user. You'll need to install/configure GAM for both workspaces.
Good luck!