Video quality depends on bitrate. May be they are transmitting channels at same bitrate as earlier or higher(as you notice better video quality) and just change the compress method to MPEG4 though Mr Bhat has just confirmed channel not yet converted to MPEG4.
Thanks Dipayan. Can someone let me know whats the old and new bit rates after MPEG4 Conversion?

Both in buying as well as monthly as no HD access fee needed.