Cities usualy change in time. It`s because people change, habits change, the level of human needs change. Major changes can take place in the city and must be neutralized for the stability of life and all of its domains.
Recession and low funds now reorientate urban planners more to regeneration and functionality conversion/convergence by using the current existing infrastructure and buildings above it, than erasing everything, planning and renewing.
Parts of the city deteriorate without even major changes, we don`t like those places, we don`t feel confortable on the street or inside the house. The environmental ambient isn`t as attractive as it once was for certain reasons, and finding those reasons and more, getting solutions to fix them is the hardest challenge. Like building a supercar from scrap, using lesser components makes the satisfaction bigger.
If you know some examples that match my description, throw them in so we can learn from others experience ! I`ll post some a little bit later !