I'm not a structural engineer either. I assume burying the existing retaining wall behind a new retaining wall would make it impossible to maintain or monitor. So the new retaining wall would need to be designed to replace the old one.Maybe this is demonstrating my complete lack of knowledge in construction and structural engineering.
But why would they have to disrupt the retaining walls to fill in some of the existing road on Bloor Street running underneath the overpass (the part with bike lanes) with concrete up to the level of the sidewalk? I'm not thinking of widening the entire underpass, just of reclaiming some of the space from the road for the sidewalk (and saving that for a new dedicated bike path).
Maybe someone here with more structural engineering background can comment.