Murray, no one really "changes lyrics at random" but people do make changes and usually they think they are making "improvements." If the author/master is identified, then I also agree that they should be credited, and if one is using their song commercially all efforts should be made to contact them before hand. If I am making "significant" changes to someone's song, I generally like to submit both what I think is the original and the revised song to anyone else who is interested in the song so they can judge what I have been working from. Sometimes the original author is not happy with my practice, and if very unhappy I would comply with their judgment and leave their song alone. There are always other fish in the sea.