DirecTV started with F cards, if I'm not mistaken. Then, they switched to H cards to provide better service. These lasted a couple years before the new HU cards came out. HU cards have a Football player on the front, where H cards are blue with a pic of a satellite.
H cards are easily programmed to receive illegal service, namely all the channels. However, HU cards are programmable, but not as easily and not as widespread. For the past 2 or 3 years, DirecTV has been claiming it is going to kill the H stream, which is what the H cards reads all the info from. It still hasn't happened.
So, you are probably just getting a letter to upgrade your card. I doubt your H card would go bad anytime soon (b/c so many people are legally using them), but stepping up to an HU card wouldn't change anything for a subscriber.
Although, they may have a new card now, so don't quote me on it.
If you don't have the new card yet, just call DirecTV. They will be happy to send you a new one.