Look at the contract you signed - did you agree that you weren't able to cancel this deal? If you agreed to a deal and now want to negotiate a new one, the parties are free to get the best deal they can, but unless or until then, the old one will be enforceable.
PLEASE READ THIS BEFORE YOU COMMENT, EMAIL ME OR PHONE ME. I'm only licensed in CA. This answer doesn't make me your lawyer, and neither do follow-up comments and/or emails and/or phone calls, and you shouldn't expect me to respond to your further questions if you haven't hired me. We need an actual agreement confirmed in writing before any attorney-client relationship is formed. This answer doesn't constitute legal advice, and shouldn't be relied on, since each state has different laws, each situation is fact specific, and it is impossible to evaluate a legal problem without a comprehensive consultation and review of all the facts and documents at issue.
While I agree that a high percentage of folks in the used car sale business are indeed crooks, there is nothing you've indicated in your post that makes me think the folks you refer to are crooks. What appears to be the case is your circumstances have changed and you want out of your deal and they prefer to hold you to your end of the bargain.
It sounds like you may wish to consider selling this vehicle on the open market and paying off the new loan. I definitely would NOT give them back the vehicle as a "repo" as they would indeed have the right to go after you for any shortfall. As for paying off your prior vehicle and/or titling it in their name, they usually do not do that for approximately 30 days. It's easier and allowable for them to just sell it as the middle man. They avoid extra paperwork and extra taxes that way and it's completely legal in most states. If they fail to change title after 30 days or they hurt your credit by not paying off the vehicle, then you may indeed have a claim against them.
Good luck with it.