As far as song replacements go, I'm just wondering what other songs would they have to replace if they were to do season sets? So far, out of all the episodes they've released on DVD, it seems like only two of them have had to have song replacements: "Theodore and Juliet" replaced "Love Potion No. 9" with a poorly-dubbed version of "Witch Doctor", and "The Curse of Lontiki" replaced "Surfin' U.S.A." with "Surfin' Safari" (which really doesn't make sense, since both are Beach Boys songs). Consider some of the other songs that are left intact: "Snow Job" still has "Footloose", which was actually a THEME SONG for another movie (of the same name). I know one episode had "Puttin' on the Ritz", which would probably have to be replaced, as I've heard the estate of Irving Berlin is nasty when it comes to people using his songs... I heard one person actually got sued by his estate for quoting a verse from one of his songs in a book of his.
I've also wondered in the use of "The Girls of Rock n Roll" was a song replacement in "Bigger!", it's really hard to tell because the mouth/lip sync animation is so inconsistent that it doesn't seem to exactly match the lyrics, but otherwise, everything else does match with the timing/tempo of the song.