You don't get demand "refunded" when dezoning. Demand will need to build up naturally again. I learned that the hard way in one of my first cities trying to switch a large area from low density to high density. :S
Make sure you don't have any of these broken mods subscribed:
- Fine Road Anarchy (original, version 2 is fine)
- Fine Road Tool (original, version 2 is fine)
- Road Anarchy
- Road Anarchy 2
- Advanced Road Anarchy
- Sharp Junction Angles
- Low Bridges High Dams
- Bridge Fix