You could be landing someone with a whole heap of trouble is you do what you suggest - should you default on the repayments, the finance company would seek to repossess the vehicle and would take it from who ever you sold it to - assuming that the car is the collateral.
There are two types of finance - one is secured on the vehicle, the other isn't. If you had to give you registration when getting the finance, then the vehicle would usually be the collateral - the likes of Opel Credit, Ford Credit, GE Money. If this is the case they may be willing to change the registration that they hold but would be unlikely to do this is the replacement vehicle costs less.
Hope that helps.