I think if you go into Device Manager and disable (not uninstall) your video card, then your system should default to the on-board video (assuming you have those drivers installed).
Depending on your motherboard, you can just boot into the BIOS to select either. Don't forget to change the monitor cable between each video adapter though.
just to add to it, disable the onboard after removing from device manager, probably would be a good idea to remove drivers as well. Boot to the new device and install its drivers.