The fellas over at TechPowerUp have posted a guide detailing how to mod a Radeon HD 6950 (Cayman Pro) videocard into a Radeon HD 6970 (Cayman XT), both of which are based on the same GPU design. The main difference comes down to the number of shaders -- 1408 on the 6950 and 1535 on the 6970.
According to TechPowerUp, the 6950's missing shaders are the result of a deactivation scheme in the card's BIOS. That's good news for modders, as unlike hardware locks, software locks are usually easy to reverse.
Bear in mind that you're on your own should something go wrong, but according to the guide, activating those dormant shaders involves nothing more than flashing the BIOS. Once you're finished with that, you can goose the clockspeeds to match that of a 6970 videocard.
TechPowerUp tested its method on 14 cards from 7 different vendors and in each case determined that the mod "unlocks and works fine" without any errors.