If you have extra disks lying around, it's pretty easy. I've toyed with it at home, although never completed it. It was on a machine with a 2 GB USB stick for
/ and
/usr with pretty much everything else on a ZFS pool.
Download the 64-bit sources, compile the 64-bit sources, installworkd with DESTDIR pointing to the new disk, installkernel with KODIR pointing to the new disk, reboot to the new disk, without the ZFS pool imported. Make sure everything works. That's as far as I got, but the theory is that you'd import the pool to a new root, make a list of the apps installed, do some ZFS trickery to save the old data and create new filesystems, and install the 64-bit versions of the apps.
At the time I was playing with it, an up-to-date KDE install was a multi-day affair (pkgng wasn't out yet) so never finished the process. It's not all that different from a backup, re-install, restore process, except the "backup" is live.
Could probably do some trickery with
freebsd-update(8) to install onto a new disk, and then use pkgng to install the binary packages. I'm still tempted to do this at home, as running 32-bit FreeBSD with ZFS and only 2 GB of RAM can be touch-and-go sometimes.