I did this on Saturday at ORD on an award ticket as an EP so should be possible unless it's dependent on status. Had a final leg on an award ticket for ORD-LGA that was delayed by 2 hours, so went to the customer service desk at check-in to request if I could be placed on the ORD-JFK that was not delayed - went pretty seamlessly and given the delays that happened at LGA and ORD that weekend ended up savings 4 hours.