I crossgraded my VPS last week (Percona XtraDB cluster wouldn't rejoin, version was too old and they've stopped doing 32bit builds). It was...an interesting experience. Not bad, just mildly challenging.

Once you've added the amd64 arch and installed the kernel, you can just shutdown the VPS, change arch in Xen console, and start the VPS. You'll then be running a 64bit kernel and a 32bit userland, which is fine.

I then went through a process of listing the remaining i386 packages ("dpkg -l | grep ii.*:i386" IIRC) and then "apt install" bunches of packages - they'll pull in the required amd64 libs. Run 'apt autoremove' regularly to clear out the unneccesary i386 packages. Eventually, you'll be totally amd64 and you can remove the i386 arch from APT.

Don't try using aptitude, it'll totally freak out. :D You should at the end, just to keep the package lists clean - any "broken" packages just need their dependencies marked for install.

You shouldn't need to do this all in one go (my crossgrade was spread over several days) as it *should* be okay to run in a mixed environment (anyone running Steam or the old Skype for Linux? That was a 32bit app running on 64bit systems).