I want to upgrade from Zimbra 7.1.1 32-bit on Ubuntu 8.04 to Zimbra 8.0.0 64-bit (of course) running on Ubuntu 12.04. I was hoping to do this in one shot with the directions here:

Network Edition: Moving from 32-bit to 64-bit Server - Zimbra :: Wiki

However, the LDAP instructions fall apart, as the database has moved from bdb to mdb format. So I got to thinking about how this should work. It would seem that I need to do this upgrade in 2 steps, one for the move to 64-bits, and one for the upgrade to the latest version.

I am really trying to avoid taking down our production system as much as possible. To that end, I'd like to bring 7.1.1 64-bit up on the new server, and follow the move instructions, and then do the upgrade. However, the latest Ubuntu supported by 7.1.1 is 10.04. Maybe running it on 12.04 is good enough for the hour it will take to get it going and then upgrade it; maybe not. Even if I were to upgrade the old server to 7.2.1, there's no 64-bit version for Ubuntu 12.04 to install on the new server anyway. There's no old 64-bit Zimbra version on Ubuntu 12.04 to match to an old 32-bit version supported on 8.04, or even 10.04.

What's the best path forward for my situation?