Apple really does not have a good workflow for migrating devices from one MDM to another.
Apple recommends you issuing a remote wipe command from the old MDM after the devices are pointed to the new MDM in ABM and have them enroll with Automated Device Enrollment. (the only option that does not require the user to have admin access)
If you remove the MDM profile and wait on Declarative Device Management to force the new MDM enrollment, devices are wide open and unmanaged until that process starts. I'm not sure about your environment, but that would not fly in mine.
Your other option is to manually enroll and use Sudo profiles to change the enrollment type which is very hands on.
