Yes, this... Even as a Presidents' Circle I would be over the moon to get an upgrade from a Chrysler 200 to a 300!

However, if you have Amex Plat (the full blown charge card - not the various platinum credit cards) then you get free 5-Star membership, regardless of your number of qualifying rentals, or at least you used to. However, it doesn't automagically happen, you have to activate it.

This is independent of the CDP, as long as it has been activated. If the OP has done this then (s)he should be 'entitled' to an upgrade, although it is subject to availability and not guaranteed.