I'm pretty sure by digging down they would be adding a second elevated seating (deck) level. Also talked to an architect who is very familiar with the structure. Thick Interior columns were added to make the Project manager at the time feel better about the support of the structure. But in reality the structure was designed so that everything including interior columns can be removed because the outside trusses keep the entire structure supported.
Its all about how radical the renovation is going to be and how much money they will be willing to put into it. I could easily see the scope at 15,000 seating.
Currently there is alot of wasted space