There are ways, but it is painful. You do not want to do what I’m going to post below.. Really. 
1) buildworld/buildkernel
2) turn off swap (either swapoff or reboot with no swap)
3) newfs /dev/ad0s1b (or wherever)
4) mount /dev/ad0s1b /mnt
5) cd /usr/src; make installworld DESTDIR=/mnt
6) cd /usr/src/etc; make distribution DESTDIR=/mnt
7) cp /etc/fstab /mnt/etc/fstab
8) copy your 64 bit kernel to /boot/kernel.amd64 or wherever you like
9) reboot
10) break into loader prompt
11) load /boot/kernel.amd64
12) set vfs.root.mountfrom=“ufs:/dev/ad0s1b“
13) boot -s
14) now you’re in single user on a small mini-install in your swap
15) mount your original file systems under /mnt. all of them. you’ll have to do this part by hand. read the /etc/fstab for the list.
16) up till this point, you are not committed. #17 is the point of no return
17) you will have to mess with symlinks to make this work. but what you want is:
cd /usr/src; make installworld DESTDIR=/mnt
cd /usr/src; make installkernel DESTDIR=/mnt
you might need to use ‘make -k’ to get this to run through completely. The problem is that you built with srcdir=/usr/src and objdir=/usr/obj. So the obj files were in /usr/obj/usr/src/… and now they’re in /mnt/usr/obj/usr/src. Since the installworld is running from /mnt/usr/src, it will expect to find its obj files in /usr/obj/mnt/usr/src. You can try a mkdir -p /usr/obj/mnt/usr and then symlink from /usr/obj/mnt/usr/src -> /mnt/usr/obj/usr/src. Experimentation will be needed.
18) reboot
19) make sure you’re in the 64 bit kernel and boot. Cross fingers 
and everything else that can be crossed.
20) just to be safe, do another installworld while in 64 bit mode in case something got skipped due to ‘make -k’ mode.
Warning: I probably forgot some critical points. I did this sort of thing a few times about 6-12 months ago and my instructions above are entirely from memory. If you try it, you will need to be able to work your way out of the mess if you get stuck.
If you try it, you’d better have the reinstall CD handy. You might still need it. You can hose yourself.
If anybody would like to try a 32->64 transition on a fresh i386 install and take proper notes for a howto or a walkthrough, that might be a good thing.
Since you’re presumably running on kernel.old right now, make sure you save it somewhere safe that you can get to.
The reason why it hangs are for many reasons.
1) our syscall translation is relatively primitive. It works pretty well for 32 bit application level programs, but not for system
programs. eg: mount, init, etc. sysctl is broken for 32 bit binaries because their idea of a ‘long’ doesn’t match the kernel. The mount interface has got binary blobs of data being passed around that we do not translate.
2) Right now, 32 bit binaries use the i386_userldt() facility in their early startup, which the amd64 kernel does not implement. We have to use a custom 32-bit build of ld-elf.so.1 and libc.so.6 that implements the thread-local-storage features via mechanisms that are available. So what is most likely happening is that /sbin/init is hanging or locking up, and as a result you see nothing.
3) the 32 bit syscall layer isn’t bug free. I’ve run into some apps that should work but don’t. Obviously things are not right yet.