I often write new music for old songs, which I register as "arrangements" under copyright. Likewise when I write new words, usually for tunes that never had them, I register only the lyrics. I don't think I need anybody's permission for that in the US. I never replace another lyricist's whole lyric, but I often change a word or two here and there to suit my singing, but I never try to register that, obviously. What if I write new lyrics to a tune by a composer who has already been dead for 70 years? Could I register copyright for the whole song?

Chet W.