If you want to use the "nothing new" argument, try this:

At the outset, you have a one in three chance.

When Monty opens one of the doors, you are correct that you still have a one in three chance that you have picked the car.

But there is only one other door at this point, and your door has an unchanged one in three chance of being the car. Now what are the other possibilities *at this point*?

There is only one other possibility - the other door!

So the single remaining door must have a two in three chance of holding the car!