On second look, the first tower (73 storey) has 4 below grade levels whereas the 84 storey one has 6.
I find it hard to believe that the 73 storey structure's basement. Will only be only 4 storeys below with an above grade hieght of 862ft high! Five to six storeys below makes more sense to support the height of this tower if it's only being supported by itself just a thought!
...and I believe the only reason to dig/build floors lower than four here would be to create more parking spaces. And not to strengthen the foundation of the building instead...as one would think that building something taller on more empty space would have the opposite effect of strengthening something. >.<
Looks like they are carefully dismantling the existing structure one floor at a time. Will be interesting to see how this site operates in a pretty high traffic area when construction gets underway
What is the usual procedure for constructing a building with a historic facade?

Do they dismantle the walls and then reassemble once the construction is a bit further along? Or do they just support it and leave it as is as they are doing construction?
I assumed they are removing what didn't make the heritage cut here.