I concur that you may want to check with the branch manager on this one, as it may be at the discretion of the location.

I had something similar happen when I used free days at SJC to rent a convertible. When I arrived into town, it was raining (and it was forecasted to continue for the entire duration of my rental), so the ES agent let me take a K900 or E-Class instead (and she charged me the difference in the "quoted" rate so I only paid the $2-3/day difference for the specialty class rate over the convertible rate).

That said, a coworker booked luxury on free days at DFW and wanted to try the K900 and both the ES agent and the manager quoted him the $25/day upgrade rate on the tag (the manager insisted that it was the same charge whether he booked at the midsize, luxury or any other rate).