I guess the first question is whether the song is still under copyright. If it is, then whoever holds the rights could at least be consulted. Are the people responsible for the recording contactable for information? The names given for authors may be pseudonyms, especially if they don't seem to have written anything else - maybe Mitchell wrote it himself but wasn't sure what response it would get, because of the very issues you raise. Often professional songwriters are writing to deadlines and push out unfinished songs because there's no more time but they're under contract to produce something.

We certainly make minor adjustments to songs if the original makes little sense. If the author is known, we do try to contact them. With my songwriter's head on, I would rather someone changed one of my songs a little than didn't sing it at all. The worst thing that can happen to a song is to be left all alone on a piece of paper.

As for being acceptable, you should know by now that you will receive the whole range of responses, from those who agree fully with what you're doing to those who condemn you for all eternity for changing a single letter. Life's too short to worry about it. As far as I'm concerned, what matters is whether the people you sing to and with enjoy the songs you sing. Your mileage may vary.

Andrew