I had a 3.5 image on a pi2, did a script update (restart) then updated binaries (restart) then did an apt-get update, followed by apt-get dist-upgrade. Restarted. Worked great. Swapped to pi3 and still worked great! What’s more, a few MAME roms that had performance issues on the pi2 were much better. For instance, the music on Golden Tee 2k is pretty good now and no bad slowdowns.
I am also setting up a fresh install of 3.6 just in case. Seems cleaner that way, but I am happy I have my existing image working on the pi3.