[['These are the notes you play, AND YOU DON'T PLAY ANY OTHERS!']]
Of course, that's not saying anything about the ORDER in which you play them ... ; D

I'm with Midchuck and Deckman:

If the "folk process" had been eschewed by our ancestors, we wouldn't have 17+ versions of Raggle Taggle Gypsies/Blackjack Davy/Whistling Gypsy/Gypsy Rover (etc.) How boring would that be?

If the song's author is known, and still under copyright, you'd better get permission from the author and/or copyright owner. (I'd add that if you deliberately change a song of known authorship, even if the author is dead, the respectful thing to do would be to ACKNOWLEDGE that you've altered the original work (especially if you've put the song on a CD or in a songbook). Otherwise, you're falsely attributing words or melody to the author.
I think this is especially important if your "revision" loses some of the poetry of the original work or if you've given that work the "Rise Up Singing" or "Unitarian Hymnal" "politically correct" treatment.

"Rise Up Singing's" habit of changing lyrics to make them more politically correct or "new-age-y" without even acknowledging the changes really annoys me too.


But as Deckman says, if a word or words in an old song are so archaic or regional that they wouldn't be understood or if they are terms that would be very offensive today, I see no problem with adapting them.   It's not always possible to explain the historical context (e.g., that "darkie" was once "the politically correct term") but I would hate to throw out all the American songs of the early 19th C. because some of their original language might be offensive today. Similarly, I don't see a problem with taking a Robert Burns song and changing a few Scots dialect words to words that American audiences would understand, especially if it's a sing-along.   

But when I pass the lyrics to a song on to other people (e.g., on the internet or by printing a lyric sheet), I try as best I can to find and use the original, accurate lyrics (if such a thing can be determined), rather than just printing them out from my faulty memory or changing them arbitrarily on my own.