Upgrade from i386-8.0 to amd64-8.0 possible?

[LoH]

I've got a system currently running FreeBSD-i386-8.0, and was wondering 
whether or not it's possible to move the system to FreeBSD-amd64-8.0 
without bringing it down for more than a reboot or two (and avoid 
reinstalling all of the client software on the box itself).

The box itself will be undergoing a hardware change from a dual xeon 
(P4, not 64bit) to a dual opteron. I think I can boot i386, set up a 
amd64 cross-compile, then compile a new kernel with it, or do a binary 
change to the new arch and then reboot.

--Joseph Lenox