Yes you can trade it in even if you're still making payments on it. I had an Avalanche only for a year and a half and still had three and a half years left to pay it off and I traded it in. Basically what they do is give you an amount they think your car is worth....which will be MUCH less than what you want. So lets say they give you 25k for example. Call your finance company and find out what the payoff is on your car. Let's say you owe 26k. In that case, you will be upside down by 1000 dollars. That 1000 goes on top of the price of the G35. If they give you more than what the payoff is, then that is like a down payment on the G35.

 

I had a similar situation as you. I leased a VW Jetta for 3 years. I was not even one year into my Jetta lease when I wanted to get (and did get) my G35 Coupe. What happens is, every leased car has a buy-back value that is constantly changing (changes like every 2 weeks). Now, at the time, the buy-back value for my Jetta was $17,366 and my Infiniti dealership only gave me $14,000 for my car. So I had to give them my car and pay the difference. In the end it went down like this:
I gave them my Jetta, $3,366 (for the difference of buy-back and trade-in of my Jetta), $630 (for the first month's lease payment of the G35C), and I was free and clear from my Jetta lease and in my brand-new G35 Coupe. =)

 

i want a honda accord!!! ill trade u for my ivory G 5AT =)