The songs will change. They already have - the thing about folkies singing the Beatles, they tend not to try to sound like the original, and they don't. Over the years that difference will grow, and the words people will sing will change too, because otherwise they won't match the language people use when they talk.

I suppose you'll get purists who will try to insist they have to be sung the way they were sung to start with, but nobody will take too much notice of that.