Originally Posted by Zaynab

Yes, you are entitled to same day standby (assuming this is all on one ticket). Unfortunately, the only direct option for your 1:45PM arrival is AA 2321 at 3:40 PM, which does not have MileSaaver availability.
Both AA 2321 and AA 2323 (4:30 PM) have coach Anytime availability.
If you are okay with a stopover, you can fly DFW-PIA-ORD on AA 3246/5304 at 3:30 PM. Availability: T6 (coach MileSaaver). There are later options via CVG and LIT as well; these too have availability.

As EXP on an international trip you will have lounge access, and while DFW does not have a Flagship lounge, they do have really nice showers (one of the best perks of status, especially after a long flight).

So yes, you can be in Chicago on 2/27, either via same day standby, or if you accept an extra stop over you can change your MileSaaver ticket right now (which is free for EXP).

Assumption: AAdvantage award is coach MileSaaver. If you are travelling on an Aanytime award, space is wide open.

If you want to check before calling the EXP-desk: ExpertFlyer (which is where I got this info) has a five day free trial.