That's about 25% of the reason. The other 75% of the reason is--as bmw745ion19s mentioned--just because they can.

I would take a guess that your chances of upgrading to the Challenger for less than the reservation are about 50%--if they have one in stock and available. The commissioned sales agents have some flexibility with the price (even if they claim they don't), so remember these key points: don't act too desperate for the car, and make sure the agent knows that price is a major obstacle for you. The attitude of, "It'd be kinda nice to drive, but I really can't afford to spend [much] more than I'm currently spending" is the best one to approach a sales agent with to get a reasonably-priced upgrade. If you look too desperate, the agent knows he can extract more money from you, and if you make price an obstacle, he'll be more likely to lowball the offer or negotiate with you to get the price down.

Or, since you're from out of the country, I assume you'll probably need at least some of the insurance options (unless you have found an all-inclusive rate); if that's the case, you could try the approach of "Since I'm helping you out with a CDW sale, could you help me out with a Challenger?" Or try the tactic that a friend of mine has had some success with: "Well, I'd like to drive the Challenger, but I was also considering buying the LDW, but I only have enough money in my budget for one of those. Could you make me a deal on the two together?"

Also, there are some coupon codes out there that supposedly work on some of the Collection cars, but I don't know if they are still valid and/or will work in this particular case. Might take a few bucks off, anyway...