When AMD came out with the 6950, they were identical to the 6970. They also come with two bios's just in case something got messed up in the flashing process. By flashing the card, you unlocked all the removed things, and basically had a 6970 instead of a 6950. None of the 1GB cards supported this however, and now days AMD has fixed it so there is nothing left to unlocked. (single bios on the card as well.) Unlockable cards still go for lots of money on the used markets, but you're usually better off just buying a 6970.
For the record, you've never been able to unlock a 6850 to a 6870.