I didn't see this thread first time around. If I had my response would have been similar to the following:

I heard a song (a filk, but that's another matter) done for the 2000 UK filk convention. I loved the original, but . . .

The opening lines (and whole premise of the song) was:
"The twentieth century's over, alas.
Oh shed a tear for the days that are gone."

The parody was not long in coming, and matched the number of verses, and most of the rhyme scheme. The parody started:
"The twentieth century's over, Oh no.
For pedants will tell you there's one year to go."

The biggest belly-laugh this got when I performed it in a circle that evening was from the original writer.