Uninstall/Reinstall is the only supported method.

However:
I would love to see multiple "tenants" under a parent "org" where much of the Settings configuration could be set once used by all the tenants. Then a device could be assigned to a specific "tenant" through a client config setting. I thought I had submitted that to my account team as an enhancement request, but I can't find it on my list. Must have only mentioned it in passing.

The only 'potential' workflow outside of removing/re-installing clients would be to use IDP mode and unenroll the admin user to see if this triggers a new enrollment to the other tenant, downloading branding/config files etc. This hasn't been tested but could be a potential workaround if such workflows are permissable