Sharyn,
if the writer is not credited at all it is plainly wrong and should be corrected by anyone who is present and in the know. If an altered song is credited, then the performer again should indicate that this is their arrangement or their interpretation of the original.

Clearly our attitudes are diametrically opposite on the subject of the performer's freedom to alter a song. For myself, I am happy to have others alter my songs, lyrics or even tune details, to fit their abilities better (some have difficulty pronouncing certain words, some cannot reach certain high or low notes). Indeed, this has given me much pleasure in discovering the different possibilities in my own songs, which I was not aware of when I wrote them. And in a few cases I have even modified my own performance of the song to adopt a change introduced by someone else, because I think it is better.

I don't thing there is a "wrong" or "right" attitude to take on this, it is a personal matter and that is all. I am aware of a few well-known songwriters in the UK who are adamant that they don't want their songs to be altered in the least, but the fact is that here they have little control over that - their copyright is protected, but the precise detail of their song is not. They would have to take a performer to court to obtain an injunction against them singing an altered song, and their chances of winning in court would be very, very slight.

For example, look at the different versions of "Yesterday" sung every day by performers or revellers - if anyone could take the "offenders" to court, Apple Records would be the most able; yet they don't, perhaps because like me McCartney doesn't care, and certainly because they would be unlikely to win. And this is a good thing in my view, because it allows the "folk process" to continue thriving in this country.

I think our difference of opinion might disappear in cases of extreme alterations, but here I am referring to the normal slight modifications of songs that one encounters.