@TheWizard - The ability to differentiate users from different domains and assign them to different GUIDs during JIT provisioning largely depends on the capabilities of your Identity Provider (IdP) and the configuration options it provides. Some IdPs may allow you to set up rules or policies that can differentiate users based on their domain and assign them to different GUIDs or groups accordingly.
If your IdP supports such functionality, you could potentially set up JIT provisioning to differentiate users from Domain A and Domain B and assign them to different GUIDs. If not, you might need to explore other solutions or workarounds, such as manually managing the users from different domains until you are able to merge the domains and change all users’ emails.
It’s recommended to consult with your IdP’s support or documentation for specific instructions or guidance related to your scenario. If your current IdP doesn’t support this functionality, you might also consider whether switching to a different IdP that does support this functionality would be feasible for your organization.