When does representation become art?

I recently put together a short course for a couple of people who wanted to know a little more about editing photographs. One participant had requested I show him how to substitute a more interesting sky behind his photographs of birds, particularly those in flight.

Such a substitution is easily completed within Photoshop; but should we do it?

When does the cropped out branch in the corner of the photo become unacceptable? Is unacceptability the embellishment of a feather or of a whole wing? When does a faithful representation of ‘what was’ turn into something that is a representation of ‘what could have been’? To be fair … each to his own; and I really mean that. However, should if we alter a photo make it known or can we just accept that in this digital age the alteration of a photo is something to be expected and the representation has become art? I suspect the answer lies within the use of the photograph.