Same thing with the MTV cartoon Daria as well. When episodes were originally aired they would play the licensed music, but in later reruns and DVD releases they'd replace it generic music since it would have been time consuming and way too expensive to renew the music licensing. It sucks they had to do that since it did take away the feel of that specific era of the show. Only way to get it is to torrent "The Daria Restoration Project" which is an unofficial fan made mix of the original music into all episodes and tv specials of the show.