I had asked this same question about a week ago. The answer was
1. Someone else had done it with no problem.
2. People on these boards would be very upset if they found out that you could secure a full size car and then take something they were entitled to on the emerald aisle.
I am not certain yet how that would work.
If you reserve a full size 4 door, using a double upgrade coupon, then you would have still paid for the intermediate so it all works out in the wash.
If however, you reserve the compact car and get the choice of a full size 2 door/4 door, then I could see that maybe that would be not be allowed.
If you reserved the full size without the upgrade coupon, then you are already paying more than the intermediate so why would it matter?
I will tell you that people are saying they are not seeing as many full size SUV or Mini vans on the emerald aisle like they used too.
I guess it goes back to the old saying....It couldn't hurt to ask when you get there.