It's been a while since I popped them in to check, but if I remember right, for some reason Buffy season 4 and beyond have been widescreen here in the UK since the DVD release. The show was never framed for that, and the DVDs for the US were the same as broadcast. Not sure who would have made those decisions or why.
I know Whedon and others were very passionate about retaining the framing as it was, but honestly I think you could make the show widescreen without losing the compositions. It would need care and attention to do it well but plenty of other shows have done this. Can't help but wonder if Whedon's aversion kind of screwed us. Having no-one involved in the show interested in remastering and reframing didn't stop it happening, it just got done in a shoddy fashion by other people.