I believe that ALL devices have to be erased and re-set up from scratch (no iCloud backup for mobile devices, and you have to jump through some hoops I found for Time Machine backups not to interfere with Mac enrolments into a fresh MDM) to change MDM. To my knowledge, there's no 'one-click upgrade' path from Jamf Now to Jamf Pro, and this is partly on Apple.
To help, you should do all the back-end ABM/ASM device re-assignments and set-up Jamf Pro in advance before remotely erasing your 500 devices in Jamf Now. Once done, they'll enrol into Jamf Pro upon the next set-up. Since you're fully moving to a new MDM, you'll also want to make sure you run through the 'reclaim service token' process in Jamf Pro for your current VPP token, as this will then allow you to distribute your existing apps once the devices are in Jamf Pro, but you'll need to set-up your scopings and device groups again.
If you want Time Machine backups for re-enrolling Macs, you'll need to make sure that they're all set to exclude a bunch of Jamf stuff and all profiles as part of their most recent backup, then untick the 'System' options during the restore in Migration Assistant. I found internally that this worked for us, as it was becoming a pain when users upgraded to new Macs and restored via Time Machine without doing this.
