Um, not really sure where this should go *

looks round for help* but I was reading the Fellowship a while back and came upon the song that was adapted to form Pippin's song in the RotK film.
I remember that it was said that the song was a hobbit walking song and that it was set to a tune as old as the hills (possibly on the last one). Surely then it should be a happy song, or at least set to a cheery tune, it being hobbitish and all?
So am I correct in saying the original probably was cheery and light and PJ just adapted it to fit the moment, I know he changed the words slightly but did he also change the meaning?