You can upgrade roads from one type to another and build over them -- it doesn't necessarily need to be an upgrade, either. So if you have a one way road, and then decide you want a two-way road, it's easy enough to make that change. It will automatically rezone residential, industrial, or commercial zones to fit the new road. The only difficulty is if you have a service that can't be removed (school, firehouse, etc.), then you'll need to move it temporarily away and you can move it back after you upgrade the road.
But yeah, basically, if you build an entire city section, and discover you've built it wrong, you might have a slightly difficult time -- but that's what save is for!