Some are capable, some arent. But there is no gaurantee it will work. Be on the safe side and run it as a stock 6950 or buy a 6970. If you dont have any experience with GPU BIOS's then dont even try. Most likely youll brick your vid card.
Ill reiterate what I said above. And I wouldnt trust a YouTube video for anything. If you dont know what you are doing, leave it be or buy a 6970. Some can be unlocked, some cant. There is no gaurantee and if it bricks it, theres a good possiblity you wont be able to RMA it. Id be surprise if anyone gave you tips here, but they may.