The only time that's an issue is if you owe more on the car than it is worth in trade. You have two options in that situations:
1. Put your present car up for sale yourself for however much you owe on it and hope someone is willing to pay that much for it.
2. Keep driving the car you have until you have it paid down enough that you don't owe more on it than it's worth.
Actually, $8,000 is a lot if your present car isn't worth $8,000 as a trade-in. If you look up its Kelly Blue Book trade-in value and it shows the car is worth, say, $6,000 as a trade-in, you're upside down by $2,000, meaning you owe $2,000 more than the car is worth as a trade-in, which is more than a dealer is able to wrap into the loan for the car you want to buy because a finance company will only allow a dealer to finance up to the average retail price of the car you want to buy, not more.
Some less reputable car dealers will sometimes work deals where they take a car that on paper because of its year and its odometer reading looks like it's worth a lot more than it's actually worth in order to roll in negative equity in your current car loan into a new car loan. But that never works out for the buyer because you always end up buying a car with a mountain of problems for a lot more than it's actually worth, which then you're stuck with for a very long time until you actually do pay it down to what it's actually worth.
For example, a car dealer may sell you a car that because of its year and mileage looks like it should have an average retail price of $10,000. In reality, the previous owner drove the hell out of it, smoked in it, trashed the interior, and then wrecked it, which the dealership then fixed for as cheaply and poorly as possible so that even now you can tell it's all bondo and the body panels don't meet up right and within even a year or two the paint is going to start peeling--a car that they bought at auction for only $3,500 and in actuality is worth at most $6,000 retail. One of these disreputable dealerships will be very glad to sell you that car for $10,000 and pay the $2,000 you're upside down in your current loan. That's because they'll still make $2,000 more than the car is worth retail and will make thousands more than even that profit. The problem is, you can't sell that car as a private party for retail. The minute you drive that car off their lot, the most you'll be able to sell that car for will be about $4,000 or $5,000. When all that is actually wrong with that car starts going wrong almost immediately, you're going to be stuck and totally screwed because you're never going to not be upside down in that car. That car is going to be worthless, scrap, long before you ever pay it off.