I upgraded from today. Beforehand, I created a tar gzip of the entire zimbra directory (with zimbra stopped).
Did the upgrade and, with a couple of minor exceptions it all went well. One of the exceptions was a user who could not send mail via the mta from a client (Eudora, I got the same results with Thunderbird). The logs indicated an SASL error and I searched the forum and began deleting and trying to recreate certificates.
After an hour I gave up, moved the new install out of the way (mv /opt/zimbra /home/backup) and tar gunzipped my backup. Now, when I try zmcontrol start, when it gets to starting the mta, I'm asked for a password. I'm rebooting the system right now, hoping that will fix it, but that is an old Windoze trick that I have no hope for here.
Why is it asking for a password. I thought (incorrectly, I guess) that a backup consisted of only /opt/zimbra.