?- old deck joists sit on wall plate

I am wanting to replace or repair my old deck. its about 28' x 12', 8 ft above a concrete porch for the walk out basement. The 2 piece solid beam (4 x 10's) is rotting, I was wanting to replace it and the 4x4 posts with 6x6. The decking also needs replaced. I was debating on replacing the whole thing, since the joists and railing are whats left and the joists may be soft on top in some places and the railing wouldnt pass building codes of today. The deck must have been built with the house in 1972 because the joists set on the wall plate, and the first floor joist is nailed to the ends of the deck joists. On the outside there is the plywood type siding (t111?) around the joists, sealed with caulking.
If I was to replace the deck joists, how would I do that? I am assuming the deck joists are load bearing for the wall above it. I dont know if there's blocks between the joists yet, there's insulation between them and there's only a 1/4" gap between the first floor joist and the wall plate. Also, the posts now set on cast metal bases on the concrete slab, then there's metal angle brackets screwed to the posts and concrete. if I replaced them would I have to dig up the slab (12' x 28') and put footers in first? I dont know how thick the slab is but was thinking of using the Simpson post bases with the 1" diameter bolts drilled into the concrete and post.

Any info would be greatly appreciated.