Today I installed a new Joomla 3.0 site and connected it to an old Joomla 3.0 database. Thanks to errors on a site, I ended up only with the mysql database and no joomla file installed on the server. I wanted to install the latest version of Joomla and point it to the old database. It wasn't straightforward, but it was easy. Heres how you do it. (Note: This will probably work with Joomla 2.5 but I haven't tested it). Update: I have connected a NEW Joomla install to and OLD Joomla 2.5 database and it worked! - but only after I logged in and used the "FIX Databse" button in Joomla 3 (nder "Extensions manager - Database tab).