If you have a large enough swap partition, you can do a cross-compile to build the 64-bit world and kernel, and install it over the swap partition. Then boot into that. If that works, then you can do an format and install over the real / partition.
You'll need to reinstall all your ports afterwards. Formatting /usr/local and deleting /var/db/pkg, then doing a fresh install of the ports would be cleaner.
However, after all that, you would save a lot of time and anguish just doing a fresh install off a 64-bit CD.
(Be sure to have known-good backups before starting any of the above.)
