Twitch.tv and Justin.tv are now separate sites, thus you can no longer change your category from Gaming to any other non-Gaming category, and vice versa.
If you created your account on Justin.tv and you previously switched your category to Gaming (ie, Twitch), don't worry, your account is on Twitch. The easiest way to check this is to go to twitch.tv/broadcast and click Show Key. If you see your stream key, you're on Twitch. If you do not see your stream key, your account is likely locked to Justin.tv.
If your account is on Twitch, you can only chat, follow, and broadcast on Twitch. You will not be able to login and use Justin.tv with your Twitch account.