I tried to upgrade my install from 6404 to 6500 (on the way to 6503), but it failed both times. Luckily, I had the backup folder to quickly regain my previous position.
I ensured the PMP service was stopped, and that the MYSQL process had stopped before beginning the upgrade.
The behaviour I saw was that the PMP service would restart post-upgrade, but I knew something was wrong when I tried to logon from the web. My smart card was rejected, and after guessing, I found the admin/admin default password had been invoked. The resources were all gone, too. It was like the database was wiped.
I tried it twice, even rebooting the server inbetween, and had the same result. Just for fun, I tried the upgrade from 6404 to 6500, then 6500 to 6503 all in one session (again making sure MYSQL process was stopped). Same behavior.
Any idea how to resolve this?