To reassign a SAML user from one instance to another you need to complete the following steps:
1. Deactivate the user in <leaving-instance>.zoom.us
2. Delete the user in <leaving-instance>.zoom.us
3. Remove SAML mapping (employee/student) for affected university
4. Inform the users to logout all clients and login to their own university, aka <new-instance>.zoom.us
I notice that some users who login to <new-instance>.zoom.us will get a Zoom announcement that they have been transferred to a new instance and have to acknowledge the transfer in two steps.