Update .gitmodules with the path to the new repository
Remove the corresponding line from the “.git/config” file
Delete the corresponding directory in the “.git/modules/external” directory
Delete the checked out submodule directory itself (unsure if this is necessary)
Run git submodule init and git submodule update
Make sure the checked out submodule is at the correct commit, and commit that, since it’s likely that the hash will be different
After doing all that, everything is in the state I would expect.