There is a brute-force dangerous way: overwriting the i386 versions with 64-bit binaries and libraries from another system. I did this recently; it took some iterating to get going. And then I only trusted it long enough to do a buildworld/installworld. This is not something to try unless you are fairly familiar with FreeBSD, and only after a backup. For most users, it will be quicker, less painful, and safer to follow SirDice's advice.