I guess you can argue that if it is a "written" song, i.e., one with known author and copywritten ...then any deviation is 'wrong'...but I have heard authors sing 'em a bit differently....

the answer is, of course, that it is highly subjective....I tend toward being a purist snob and not liking 'many' changes to old favorites, but even I have messed with songs at times because I just think there's a better way...or I simply mis-rememberd some minor phrase. I do NOT usually make gratuititous changes which will make it hard to sing in a group that 'knows' it.