I'm guessing that Aizen's transformation was not just to be stronger but to also make him able to be the new Soul King. Since the Soul King is the linchpin of the three worlds, I'm guessing he'd need a connection between them. Using the Hogyoku to bridge the gap between Soul Reaper and Hollow made him in tune to those worlds and able to be SK.
And speaking of worlds, what about hell? Is it affected by all this?