The following actions should be performed:
1. Deassign Plesk Control Panel server from CMail without removing data (leave Remove all mail accounts from the mail server unchecked).
2. Migrate data from old CMail server to new CMail server using Plesk Backup Manager.
3. Assign Plesk Control Panel to new CMail server without mail content migration (checkbox Migrate mail [...]

How to restore plesk mail backup to cmail server . only solution for now is:
You need to separately back up/restore mail accounts on the Centralized mail servers using Plesk`s backup management tools.
For example, if you use Parallels Plesk for Unix as Centralized mail server, you need login to mail server using [...]