About writing new lyrics to another tune- "Rum and Coca-Cola" uses the tune of "L'Annee Passee". The only thing is that if someone wanted to write new lyrics to the "Rum and Coca-Cola" melody, they would probably be sued because it was copyrighted (by someone who didn't even write the song, just rewrote the verses!) so if you want to use another tune, use a traditional one.