Originally Posted by scubaccr

NO HURRY !, you can always purchase Ambassador membership at any IC hotel during checkin.
(IC-PLG tends to upgrade Ambassadors even on award nights anyway)

IC's have membership sales targets, so ask politely during checkin something along the lines "If I buy Ambassador now, what room upgrade might I get".

Asking during checkin means you know you will be getting upgrade else you don't buy on the checkin day, plus because hotel has sales targets, IC-PLG may provide you a nicer room that buying online before the stay, eg an external view rather an inner courtyard view.