That would be a nightmare. Lowering the elevator shafts wouldn't be easy. Nor would the escalators.
More likely, they'd fill up the tracks to the current platform height, and simply spec vehicles that would fit in the resulting tunnel.
Or do what Brussels does, except in reverse: lower the boarding areas and have steps and ramps up to the elevator/escalator level. (Photo by me.)
Reverse, because Brussels builds its pre-metro LRT stations to allow for future high-floor metro trains, Sheppard would be the reverse.