I believe only those who own the 501/508 are exempted from the DVR fees but lately, the shows are moving to HD. Therefore, 501/508 will not be usable on those HD shows (MPEG4?).
Has absolutely NOTHING to do with either HD or MPEG4 - it's because in the near future, they are moving ALL SD channels to 8PSK - which those 2 DVR's will NOT receive. (along with several other non-DVR receivers) They are now replacing broken 501/508's with the 512 DVR, which is ALSO an SD-only receiver. (that does 8PSK)