32->64 bit upgrade

[Neil Booth <neil@xxxxxxxxxxxxxxx>]

I have a 64-bit CPU but am currently running 32-bit dragonfly.
Is it possible to do a buildworld and buildkernel to upgrade to 64-bit
in-place?  Or does it necessitate a complete reinstall?

I'm concerned about e.g. the new 64-bit userland overwriting the 32-bit
one whilst still in use before installing and booting the 64-bit
kernel, etc.  I couldn't find anything about this topic on your webiste.

Neil.