Classic Storage Account with large VM VHDs / Migration to ARM

Hello all. I was tasked with ensuring that all of our Azure VMs were being backed up. Seems easy enough until I went into a subscription that didn't get much attention. There were 5 vms that weren't backed up. I set up an Azure recovery vault, created the backup policy, and tried to add the 5 vms.....Failed. It turns out that these 5 vms had their disks stored in a classic storage account and Azure backup won't back them up. In looking at all of the resources in that subscription, it seems that all of them had been migrated from ASM to ARM except the storage account. Not having migrated anything from ASM to ARM, I read that migrating the classic storage account can be done by clicking on the Migrate to ARM option listed in the storage account. Now the question(s): Each of these 5 vms that are using the classic storage account for disk storage have three or four disks and each of the non-os disks are pretty large; over 500 gigs. Using the 'Migrate to ARM' function, what's going to happen? Do i need to deallocate the VMs? Will it migrate all of the VM vhd disks to a new storage account and update the VMs so that they know where the migrated disks are? Do the migrated vhd files turn into managed disks?  I was just looking for some details on how the migration plays out because it would be a monumental task to rebuild those 5 VMs with no backups.  Thanks for reading this and any feedback would be much appreciated.