Migration went well. It took me about a week to figure out a way to move the data without losing everything, but we're up and at it this morning with no problems.
Basically I followed this procedure :
Network Edition: Moving from 32-bit to 64-bit Server - Zimbra :: Wiki
And instead of entering "zmcontrol start" at the end, I had to go back and do another install and choose the upgrade option (although 6.05 was already on the system). It didn't eat any mail and all the SSL stuff cranked right up afterward.
If I tried to run "zmcontrol start" at the end of the document above, mailboxd failed and there were numerous errors in LDAP and SSL.