John, I totally agree with what you are saying about improvising by means of chord substitution, adding chords, adding 'quotes' from other songs etc., in fact, my accordion teacher would do the same thing when teaching beginners simple songs just to show what could be done ...
BUT, to me, those 'different' chords are still the 'correct' chords in that they follow a certain structure ...
Perhaps we are just hung up on semantics ...