I'm using Zimbra 7.2.7 on a 32-bit CentOS 5 box. I desperately want (and need) to upgrade to 8.x on 64-bit CentOS 6. It looked like the migration tool would be ideal for this, but when I install it, Zimbra crashes. The error in mailbox.log is Java.lang.OutOfMemoryError: PermGen space. This makes me suspect that I don't have enough memory assigned to Java.
I've looked at the Zimbra Wiki article, "Performance Tuning Guidelines," as suggested here (and elsewhere). That article says that, on a 32 bit system, mailboxd_java_heap_size is limited to 1.5 Gigabytes. I dumped my config with zmlocalconfig and sure enough, mailboxd_java_heap_size = 1420. Probably not enough.
I tried the migration tool last year when Zimbra introduced 8.0 for 64-bits only, but had the same problem back then.
My server has 16 gig of RAM, running the PAE 32-bit kernel. If I can just get the migration tool running, I will upgrade it to 64-bit and life will be good. 
Any suggestions are greatly, deeply and profoundly appreciated.
-- Stephen