Congrats! I don't know the car seats rules there in Texas but why would you trade in a "paid off" car esp. it's a Boxster to the dealer? The dealer might lo-ball the price value of your Boxster.
I don't know your financial status, but IMHumbleO...I would keep it and get a family car or sell it and then use the money to buy a family car. If you are going to sell it, wait till probably around April or May so you could get more money out of it
I give you a pat in the shoulder for thinking/preparing ahead about the new addition to the family.