You can download Microsoft Certified USB UART Driver from the link below and try using it on Windows 8.1
Also, please have a look at the thread: Use of the Cypress Microsoft Certified USB UART Driver which mentions that:
"For Windows 8 and Windows 8.1 you cannot use the PID 0003 for the USB-UART example project for PSoC. It will be good if you use the PID as 0008 instead to bind your device".
Success! Thank you Ekta!
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?