I filled the grid fairly quickly last night, but I thought I was going to need a lot of hints for the endgame. I decided to sleep on it, and as I was lying in bed, how the switch in 38 worked came to me. So now I have everything (word changes, parsing, and the four pairs) except something which I feel must be obvious and I'm just too dense to see. I have a few weak guesses for the six-letter adjective, but really no idea.