Since you're posting on a Money site I presume you're interested in the financial logic of changing your car.
Bottom line, it almost never makes sense to change your car for financial reasons. The only exception is if you have a car that is causing you trouble, costing a lot in repairs, or becomming unreliable.
If you are paying for a car on credit, you should consider driving it for a few years after it's paid off, and continue saving the money you were paying towards the loan. You should be able to drive the car for another 4 or 5 years and save enough to buy your next car for cash.
Most people seem to buy their next car as soon as, or even before their previous car is paid off. (Chain Borrowers).
I'd say purely from a financial perspective if your current car is running well, then hold on to it. But car buying is never purely financial, if it was there wouldn't be any '03 reg plates on the road.
-Rd