Sorry for the delay. Graduate school (and life in general) got pretty busy for a few weeks there. I should be good for a while from here, though.
Enjoy the chapter.
I haven't read the genre before, so it's hard for me to say if the intent was "whenever your heart flutters" or just that it takes effect "when your heart flutters". As far as I've looked ahead, Tsumugi doesn't switch back, but I can't say that it won't happen!