Hi,
All user accounts will be migrated to 6.3. The basic idea is the LAN Watch and Domain Watch (6.2 LAN Watch AD function) have been combine with other features into the Discovery. You current users that are logging in with AD credentials, will be there in 6.3 and still reference your AAD users for the domain credentials. However to synchronize the data I would highly suggest to initiate a Domain Watch Probe on your AD Domain Controller to make sure that Kaseya will gather the latest information.