I just went through this about 2 weeks ago. Though it is a little different, here is what they did with us.
We bought an ION2 and we were not happy with an aftermarket sunroof that they sold us and installed.
We returned the vehicle (has to be under 30 days and under 1500 miles) to get either one with a factory sunroof, OR without...no aftermarket.
There are very few factory roofs apparently on the ION2, so we would be getting the cost of the roof back (800$ Sticker).
We found that for 400$ we could get into an ION3 without the roof...so we "upgraded" to the ION3 and were due 400$ back.
What they did was flat out buy back the car for the amount of the vehicle. They cut a check to GM for the full amount of the vehicle and I had to sign the title over to them and a release stating that I knew they were buying it back.
They then took our 500$ deposit and applied it to the ION3 as we had done with the ION2, and just started over with all of the paperwork.
Basically it was as we never bought the ION2 and just bought the ION3.
Hopefully your trade has not gone back out onto the lot already or to another dealer in their loop. We found our 2001 SC2 already on the lot with a "sale pending" sticker on the window after 1.5 weeks.
HTH