About Joomla1.5 database change?

Hi, Im managing this joomla site, and i want to change the database prefix; I know i should do it on install, but in the middle of the project the client put another person and after weeks they turned back to me. Now Im closing all the possible security threats and I have this doubt: I’ve exported the db, changed the prefix in the mysql script and populated a new database; but when I was going to import it in the original database, i thought I’d better change the config file to point to the new database, thus having a backup in case something went wrong, so i could quickly roll back… but changing the database and the prefix doesnt work. “Cant connect to db” and when i looked more into it, it seems the session gives trouble… where do i have to look to solve this and go on with the change? is that easy to swap databases? the DB user in the cpanel is the same in both DBs… i dont know what im missing here