Sorry this is a bit of a read.
I am not sure about systems manager for MacOS devices as I use it for iOS devices. I use Jamf to manage my MacOS devices.
In saying that I had a user last year migrate their user account and system settings onto a personal device and that device was suddenly showing in my Jamf instance, I am assuming the same would happen with systems manager. Unlike a true ADE enrolment though I was able to manually remove the MDM profile from that device.
Also bear in mind with systems manager that you can only enrol up to the number of licenses you have, there is no grace period so you would need to have spare licenses if you plan on the old and new hardware running in parallel for a brief period.
A grace period on SM licenses is something I have asked for as replacing devices such as iPads it's nice to have them configured before removing to old units which means running devices in parallel for a short time.
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