https://learn.jamf.com/bundle/jamf-pro-documentation-current/page/Automated_Device_Enrollment_Integration.html

Thanks for the response. Alas, I hit the post button before I was able to get all the details added. For our new devices this will work, but given that we had Jamf Pro before we got ABM complicates matters.

you have to do the Configurator dance.. and old Macs are not supported.. and they all need to be wiped.. 

https://support.apple.com/en-gb/guide/apple-business-manager/axm200a54d59/web

This overlooks the fact that these computers are already in Apple Business Manager.

and your jamf server is in ABM.. ? 

Currently is says No MDM Server.

then you need to add it using the info above..

This is a very long and drawn out process. We just went through it and it took the better part of 6 weeks to complete. I wont reiterate all the details about how to sign up, but once you're done with all of that and you add in your vendor ID codes, then you can go back to the vendor and ask them to do a mass import of all of your sales. we had 3 vendors and 2 were able to complete this which gave us the majority of our macs into ABM, and of course everything going forward. At 1 of my last employers, we used one of the same vendors that we use now, with a different rep, the 1st one said it was impossible to do a mass import, the 2nd one knew better. Don't take a "no we cant do that" as their first response....

I was reluctant to accept this as the solution at first but after extensive reading followed by trial and error, this is indeed the correct solution. Thank you.