Since I already had the Cypress driver above installed, changing the VID to 8 was all it took for Windows 8.1 to automatically use the correct driver.
For anyone that comes across this thread in the future, go to your "Top Design" and select the USBFS / USB UART device. Double-click to open configuration. Select "Device Descriptor", and change product ID to "8". Make sure to disconnect the USB device otherwise the new PID won't reflect in device manager. (at least for me, you may also have to right click > uninstall device on the old USBUART driver).
Interestingly enough, the default is "F232", which I suppose is an ode to the FTDI F232 chipset?
If you like, after flashing and connecting via USB, you can verify inside Windows Device Manager.