I am a BMW select owner as well and I can tell you that the select financing has nothing to do with your ability to trade in the car. It is exactly the same as traditional financing except you have smaller payments for the first couple years and then owe a large chunk at the end (which you can also refinance).
Whatever the trade in value that you get for the car just goes to pay off the balloon and loan remainder. The only problem is that you will most likely be upside down on your current car unless you put a nice chunk down when you bought it. That is just a result of trading in a car that is only a year old. I dont know the mileage of your car but I would guess that the trade in value of your current car is probably around 60k. Assuming a purchase price of 75k with 5k down than you will still probably be upside down 10k. Put that on top of the price of a new 75k 2012 M3 and you are at 85k. I bet that even with the best rates that you will still see a huge increase in payment.