As several people have noted, Platinum status is pretty meaningless at an IC.

Ambassador status does give you an upgrade - as an Ambassador, you should expect a one-class upgrade (but may get more).

a) you could buy the status well in advance, and reserve a room one class below Club. Contact the hotel in advance or at check-in and make sure they know you prefer a club upgrade rather than a larger room upgrade.

b) or you could wait until you check in. When you check in tell them you are considering purchasing an Ambassador status and if you bought it on the spot, what kind of upgrade can they give you? I am not sure but suspect that the front desk clerks get a bonus or at least a pat on the back for every Ambassador status they sell -- and I've suggested "b" to a number of people who have reported back great results. For IHG, that's a good deal because these people become customers for life. That's how I got into the 6 Continents program in the mid 1990s and I have been spending 100 or so nights a year since then.