Please note that there are different types of migration. There is an ON-PREM -> Dedicated SAAS. The other one is ON-PREM -> Shared SAAS. When you said your devices needed to be re-enrolled, you are probably looking to move towards a shared saas. There is no migration from ON-prem to shared saas. If you want to keep what you have right now and don't want to reenroll devices, you will need to migration your database to the dedicated cloud. on the day of migration, you will just switch external DNS entries.
Anyways, I pay 3 million+ for 3 years. its worth the move if you don't want to do the hassle of hosting and maintaining your on-prem. Been there done that and i think its worth it!
