Adapting a car centric city into a walkable one is a bitch of a challenge because such cities are often zoned for single use purposes, which contrasts with the mixed-use zoning needed for walkability. For instance, in a car based city, you might find skyscrapers dedicated solely to housing, offices, or retail. In contrast, a walkable city features mixed-use skyscrapers that combine housing, retail, and office space, allowing people to live, work, and shop within a small radius.
Unless the suburbs are bulldozed and replaced with low to mid rise mixed use buildings, the transition won't work effectively. You will end up with the inefficiencies of both systems: the sprawl and dependence on cars of a car-centric city, and the lack of true walkability of a mixed-use urban area
