I just wanted to clarify that the reason they are at different levels is because the one that was existing was build too high. There is only about an inch below the door threshold.
The new one was build so that there will be about 4" below the new slider.
The reason the old one was built so high was that it was built to cover existing concrete steps. The joists lay directly on the concrete. The only way they could have made it lower was to break out the concrete. (this is just one example of things in the house that I would have done differently)
It's hard to visualize, but I like the idea of making the transition a gradual one.
My plan is to basically take out the stairs, extend the frame of the existing deck about three feet, then add the stairs back.