How can I reduce the display resolution in Windows (Boot Camp) on a Retina 5k iMac?
I was able to solve this myself by realizing: (a) Windows 8.1 upgrade had changed my display adapter to a Windows Basic Display Adapter, (b) AMD's web site doesn't apparently host installers that work for the Retina iMac (yet?), and (c) the Boot Camp package that Boot Camp Assistant adds to your Windows installer contains a version of AMD Catalyst that will work and will replace the errant driver:




The installer is in &lt;WININSTALL&gt;:\Bootcamp\Drivers\ATI\ATIGraphics\Setup.exe
Run install
Verify in Device Manager that the Display adapter is AMD... M29{0,5}X


Finally, I was able to update resolution: