The config upgrade code steps nicely through each upgrade step 1 at a time, and each step deals separately, in time order, with config format/data changes required for a particular new feature/fix. I reviewed it all "some time ago" for any bits that might go wrong, and it is all good as far is I could see. You can restore an old config from any old pfSense version (and even an old m0n0wall config) and the system will reboot and apply all the needed upgrade steps.
So choose whichever path you like -
upgrade the existing 32-bit system to 2.3.5 then save the config, install a 64-bit system and restore the config; or
save the existing "old" config, install a 64-bit system and restore that old config
either will work equally well.