Sounds like what you want to do is re-finance your car, not trade it in. Any time you spend money in a Subaru dealer, you're getting ripped off. Once you have to live with but what you're talking about is going to cost you a lot of money for little gain. If you want to lower your monthly payment, talk to a bank. They can pay off the subaru loan and give you a new one. A 09 car you should have no problem getting a 4 year loan in the neighborhood of 5% interest if your credit is decent. If you had, say, 10K left (sounds like it may be even less than that?), that would put your payments at $230. Basically, $23 a month for ever $1K financed.
04 STi 16G powered lagless fun
04 WRX: SOLD