The system does not permit the transfer of existing groups to different root groups or hierarchies, even if there are no learners associated with the group being relocated. This behaviour is by design.
When attempting this action, you will encounter an error message that reads ‘Learner groups cannot be moved between hierarchies'.
If necessary, you could consider deleting the group (if it is no longer needed), and then create a new group with the same name within the desired root group.