Q:
Changed GPU game won't start
Open %appdata%/Guild Wars 2/ and delete the entire folder. Open your GW2 folder (locate Gw2-64.exe), delete the bin folders and try again.
If it still fails, clean install your graphics drivers using an older version, run Windows update for any other hardware, and update your motherboard BIOS.
It’s a DirectX issue (d3d9.dll). Make sure your DirectX 9 version isn’t somehow corrupted; perhaps, redownload it.
Honestly at this point I’m just giving up. I’ve tried everything suggested here to no avail and frankly it’s more trouble than it’s worth.
You need to uninstall your previous graphics card’s driver, then install the one for your current card.
I recommend Display Driver Uninstaller for a 100% safe and clean uninstall, then go to the nVidia website and download and install the latest one for your current model.