Sorry i didn't post a response -- I have never stayed there. Used to stay at the Marriott Inner Harbor at least once a year, but the convention I attend moved to DC

To have a better chance of upgrading, make sure that your reservation includes the fact that you are now a Platinum.

And if never hurts to ask at check in "did I get the upgrade"? The check in clerks seem to have a lot more leeway than CSRs at the airport.