Just a personal feel about it, but I've always thought fans give Trek leeway for the transition between TOS to TMP and the TNG-era because of the realities of Trek's beginnings as a canceled 1960s network TV show with a limited budget, as well as Roddenberry being personally connected to the jump with TNG (i.e., "if the creator is ok with this...").The only difference is that now fans are convinced that visual continuity trumps all when it never has.
It was not so much a situation where they were making visual changes to explore different directions, as it was here's something the way we intended to do it but never really had the chance to because of circumstances. And I believe Roddenberry's explanation for something like the Klingon forehead ridges was that the Klingons always looked like TMP/TNG Klingons because that's the way it was intended to be.