The "tourist" vs "working" views of a city to the side, I think the big solution isn't revamping existing cities at first, it'll be creating pockets of new development with different goals.
And there's lots of weird things that affect American suburbs - minimum road width is often tied to service vehicles (specifically fire and garbage trucks); these could be overcome with some thought.
I've often wondered if a "carmpromise" city could be built - let's assume a ten minute walk is half a mile. Imagine a strip of city a mile wide where cars are effectively banned. Parking garages at the edges provide car access for those who have them, and the rest is entirely built around walking/biking. Perhaps a tram line runs through the middle. Then those who want cars (basically everyone) can move in without fear, and if some? most? eventually realize they're not using them they can save money by eliminating them.