In answer to your question, my understanding is yes, they will prorate the remaining value of your three-year membership and credit it against a life membership. Of course, you wouldn't want to just take MY word for it. But in the scenario you describe, it might well play out the way you are figuring that it could; that the price break on the life membership from the Plat level could make this little two-step into a worthwhile way to dance your way in and pick up some miles.