The answer to your questions are quite involved. However there is one over arching thing you must be aware of. It is mission critical to configure your customer portals with Android Enterprise, immediately. No one has control over Google's deprecation schedule which means that your customers are going to be exposed if this has not been vetted immediately.
The Migration paths for re-enrollment of devices which will now properly be managed using Android Enterprise functionality in MaaS will depend on which level you choose: Profile Owner vs Device Owner/COSU. Profile Owner enrollments are significantly easier, however this enrollment process may not meet the security and management requirements for each of your customers. Device Owner enrollments for devices currently enrolled in MaaS or any MDM, require a complete factory reset of the device including removal of factory reset protection. This is a Google imposed requirement.
IBM has a lot of great process documents in the Solutions catalogues. I highly recommend using these. Further if you reach out to your IBM partner rep or IBM MSP you work with, they should be able to arrange some training for you.
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