Perhaps there is some difference between a permanent recording of a song (equivalent to the published anthology mentioned by Wolfgang) and a transient performance of the song (equivalent to someone telling the story).
I can see little harm in changes made for a transient performance, but I can see that a permanent recording could be irksome if it diverges from what you intended the words to portray. And I imagine the aggravation would be increased by the fact that you can't EXPLAIN to everyone that hears the recording - so it might be a bit like the road-rage effect - and argument with no closure.
This is pretty aweful I guess for the songwriter. But I also believe that you can't really OWN a song even if you did write it. It's communication, so its subject to change and mutation as it travels, like chinese whispers, and could end up totally different from where it started. Its a signal vs noise thing - and some of us are very noisy singers...... :>)
My serious point is that if I am performing a song then I want to communicate, and that means adding something from me, not just forwarding someone else's message.
Harvey - I don't think people change lyrics with the intention of improving a song, its more a case of adapting the song. I'd like to think that the changes can't actually take anything away from the original creation.

Kris