Uninstalling the SDK is not enough, you also have to regenerate your solution file.
I uninstalled 10.0.26100.0 and left only 10.0.18362.0 installed. Beware that the Include directory does not get deleted when you uninstall 10.0.26100.0, so if you don’t regenerate your solution, the old includes will still be referenced and nothing will appear to have changed.