This is interesting and I'm just thinking out loud.....
I am not a contractor or even pretend to be one, and it's fairly obvious that you would need to have one come out and take a look at specific situation. However, I'd be very concerned on the pounds per square inch that the load carriying posts of the new deck would do to the existing spa surface.
Rather than place the new hot tub directly over the old, in-ground one, how about building a larger platform that covers and hides the existing in ground tub and is large enough to have the new spa well off to the side?
This would create a defined spa,-patio area, hide the old tub, but might not be as damaging to the structure of the in ground? Hmmmm . If done right the raised patio area could even have some form of access panel to the lower tub for matinence? Whaddya think?
Addtional concerns would involve closing the old tub. I'm sure you'd need to put some type of weather proof covering over it so it doens't fill with rain water, dirt and debris. I'd give a lot of thought to this......
Or find a new place for the new tub. Close up the in ground, build a small covering for it, and put plants on it.
