I've had a real battle with wordpress. A few months ago I tried to upgrade to the latest version, but something went wrong. So I left it. I finally got some time to look at it again, since I'm meant to be studying for SQL 2012 Certification, and got it working. It was a real battle. I know nothing about wordpress, so in the end I just backed up the wordpress mysql database, deleted everything on this host, and reinstalled wordpress
Despite seeming so simple this didn't work. Kept getting server internal 500 errors, until I finally found this post on my hosts' forums which directed me to this php.ini file that needs to be created but wasn't getting created by the fantastico setup that was being used.
Once that was working I just setup wordpress again, found the same theme, then re-inserted everything in to the mysql database from the previous databases' backup. I probably could have pointed the new install at the old db, but I was battling with the 500 internal server errors and though maybe that database was now to blame.