I still consider myself to be new at this, so keep that in mind.
Here's what I did, if I remember everything. I have a working 2.0.3 and a working 2.0.4 using separate databases. You can use the same database if you change the name of one of them. (from phpbb_ to something like forum_) I started with 2.0.3 and added some mods. I didn't want to lose anything by upgrading to 2.0.4, so I copied the whole directory. (so I have a phpbb2 dir (original) and a forum dir (copy) ) Using phpmyadmin, I dumped the database into a file. 30 users, 700 posts, about a meg. I put this in my editor and did a find and replace to change the tables from phpbb_ to forum_, and whatever other little tidbits I could find. I then edited the config.php of the copied forum to reflect the changed database name. Using the changed .sql file, I created the database on the server. Sql query from file I think it was. Bingo, I now had 2 fully functional exact copies. I tried editing my mods into the 2.0.4 files, but i got errors cause some of the mods weren't compatable. So I did a full file upgrade to 2.0.4 (modless) in the copied forum. Running the update files updated the copied database to 2.0.4 specs. Now I'm in the process of installing my mods one at a time to see which ones work and which ones don't. But both forums work fine and seperately. I also changed the cookie file names and so far haven't seen any errors or problems with logging in. When I'm done installing the mods into 2.0.4, I can get rid of the original 2.0.3 without effecting the 2.0.4. I hope some of this made a little sense... I'm horrible at explaining stuff.