My brother bought the disk version of Diablo 3 and dowloaded the game and can play online with all features it comes with. I made my own battlenet account so I can play on the same computer the disk dowloaded on but it asks to upgrade to full version of Diablo 3. Do I have to pay 60 bucks then dowload it again when its already downloaded by disk? Or just paying 60 bucks give me clearance to play online with all features it comes with as if buying and downloading Diablo 3 for the first time? Me and my brother are using the same computer to play Diablo 3.
Once installed (either from the disc or downloaded) you can play Diablo 3 with any Battle.Net account with Diablo 3 on it.
It sounds like you've got one of the "starter edition" codes to play with, which has limitations on how far you can get in the game, among other things. If you purchase the full version on your Battle.Net account, you can both play on the same computer without having to reinstall.