Live performance is one thing- so many of us have to substitute quickly if a word disappears- and for a songwriter that's often the way of sandpapering the song- often find that after 6 months when i return to the master copy first sung quite a lot has changed!
As long as the new singer correctly attributes the song it's OK imho, but i do agree that changes often put back in what people expect to hear rather than the surprises you've written in... however, recently a song was corrected geographically after 30 years by the recording artist... you can then claim it's a folk song of course, but that blurs the PRS thing in the UK... i think most people act honestly in this respect and give credit where it's due, and let's face it, 4/5 of the song at least is as per original or no-one would recognise it... and if it's a commercial undertaking then the author ought to be asked if changes are to be made and can respond accordingly... without poeple suggesting they're prickly if they insist on the original... just a bit of eau-de-nil there, Mr Picasso....