I have a domain configured with 14 mailboxes (imported from Plesk). Each was storing mail locally and forwarding a copy to various external addresses.
I changed the mailbox suffix, and all mailboxes were suddenly disabled, and showing as empty (scary!). The original settings were available under Mail Aliases, with the correct forwards but the local mailboxes had the old suffix. All mail was bouncing.
I re-enabled all the local mailboxes by setting "Primary email address enabled" to Yes manually for each. The mailboxes were restored, but the forwards were lost. Luckily I had copied the alias settings into a text editor and was able to restore them by hand as well.
In the end, not a big deal. And I certainly understand that changing all the system user names under a domain is a complex task.