Changing to F lets me sing the melody more correctly. Couldn't reach the high notes in C.

I was thinking the same thing as Don (Korg80), but then your post led me to believe you changed the key DOWN to F, not up ...
Can't wait to hear the 'F' version because I really liked the 'C' version ... I really enjoy the way you don't absolutely stick to the original melody ... you just sing the song the way you feel it and make it work ...
you are a great 'listen' ...