One can change the key, tempo, the arrangement, the phrasing, but the notes are the notes and they, and they alone, define the work.
HUH??? You say that you believe you should be able to change the key, tempo, arrangement, phrasing, but not the notes? That doesn't make any sense. Because notes include some of those things you mentioned, not just the changing pitch.
Besides FSC didn't write the tune of the song. He wrote a poem that went with an old English drinking song.
I personally have no problem with musicians taking artistic liberties with a song, as long as it is still pleasant to the ear. Some of the my favorite memorable national anthem moments involved singers doing something new or different. I love hearing the anthem with harmonies. Harmonies that were not original to the song.
Sometimes when singers take artistic liberties, It doesn't alway work out that great, but I've always felt that the meaning of the song transcends how any one person sings it, and the fact that we have the freedom to sing it anyway we want to, pays the ultimate tribute to the true meaning of the song.
In my opinion, music is an art, not a science. The feeling and the message is more important than the notes.