If you already have an install using AHCI, you'll need to get the VirtIO drivers installed and working in order to do the switch.
First, add a spare disk as VirtIO (Do NOT switch your OS drive yet at this point, it doesn't need to be formatted or anything, any ZVol will work). Once booted, mount the VirtIO driver ISO, go into Device Manager and find the Unknown Device, right-click and choose Update Driver.
In the New Hardware wizard, select "Browse my computer for driver software", and enter in the mounted drive's letter in for the search path (I.e. D:\). Click Next, and Windows should automatically find the RedHat VirtIO driver. It should prompt you to allow signed drivers (if you've never installed a VirtIO driver before).
At this point, if Windows has not crashed you should be able to shutdown the VM, go into Devices, and switch the OS drive to VirtIO (you can also remove the dummy disk, we're done with it now).
If all goes well, Windows will start up, say it's installing devices and reboot back, normal as can be (except perhaps a little faster now that it's using VirtIO).