I just saw you are using Capture 1 and not LR - what I described works for LR, not sure about C1.
probably would and I like your idea better. fewer clicks or so it seems. But if he doesn't want to remove all the images associated with that keyword he has to review them anyway.
Just realized my first idea may not work since if you copy/apply you will set many things different unless there is a way to just change that one parameter. There is of course if you pull down the menu of what you are copying. But now you're really adding time and clicks
So I like Peter's suggestion better, but you may have to review them visually, and deselect the ones you want to keep with the kyword