[Jenta]

I mean you cant be writing to the same database from 2 seperate installations of vbulletin.
I guess in theory you can. One uses table prefix the other one uses a different table prefix or non at all.

Besides you would need 2 licences. One for each instance of vBulletin.

Not really sure what you mean by your second question. Document what? What hacks you have installed on 3.0? Right them down on a piece of paper perhaps. Then check them off as they are ported over to 3.5 and installed them.

Don't think this is what you are asking though.

It's best just to abandon and forget about your 3.0 hacks and start fresh. If you are not ready to take this step, stick with 3.0. It's a tough decision but I eventually made it and am happy that I did. My users miss some things but they are not dying over them.

As far as your original question - what is involved in upgrading.
You upload 3.5, edit config.php and run install/upgrade.php
Make sure you back everything up first!

After that a lot of your modified templates will need to be reverted. It's best just to create a new stock style and start make changes to it...using your old style as a reference. Then delete the 3.0 style.