I've just successfully upgraded a large ZCS 7 to 8.0.7. After the upgrade, my account count went down from 726 to 701.
So far I've located one mailbox that is no longer there, and there are probably more.
Is there anything I can do to retrieve the lost boxes? I've got a full backup from before the upgrade, but I hope not to have to roll back - just copying the entire /opt/zimbra dir took over 40 hours on a 15k SAS raid10.