I am a plat and have a megabonus certificate - cat 1-5 expiring end of March 2018. I have a couple of hotel that fit the category level. However, they don't have rooms for 3 people ( 2 adults and 1 child) available on category 1-5 certificate. Is it possible to book a capacity 2 room and then upgrade using points to capacity 3 room?
I think I tried this on and was told no by an agent. Like the 'government rate' booking code or likely (m)any preferential rates, there is a block of rooms associated with that rate class. As you might guess, the 'free night' vouchers are not the largest/nicest rooms available. In order to reserve the nicer rooms, you likely have to hope for an upgrade if booking on points or a certificate or just pay for it outright to get the guarantee.
I'd suggest calling the property in question and propose to simply pay the difference.
If you talk directly with the Premier Reservation Folks they'll help you access the information you're looking for and give you options they can see easily from their reservation system screens that we don't have access to with the normal reservation form on the website. 1-800-399-4229 is your best bet as it is the Platinum Reservation number.
If what you mean is pay for a room in cash and then use points to upgrade to a different room configuration then it may be possible but I cant remember if that is a valid use case for CAT 1-5 properties with limited room configurations. A quick call to Marriott would confirm this or not.
Good luck! If all else fails, I hear folks on this site having some luck in extending their certificates to allow for additional flexibility in bookings.