Hey Nathan,
After you have edited the shape of the fill, select the fill, and you will find 'Redefine boundary from fill' will become available on the menu as shown on picture you posted. Click this option then select the curtainwall you want to adjust and you should find the curtainwall will update. See pages 382-383 in the pdf ArchiCAD reference guide for full instructions.
To adjust the boundary of the window manually, in the 3d window editing mode you need to only display the 'Scheme' of the CW. The thick line following the edge is the boundary line, and can be adjusted exactly in the same way as any other polygon shaped element.
You may also notice that the curtainwall doesn't automatically cut its own hole if you try to place it in a wall, so you will have to come up with a solution to solve this. I'd recommend using the 'FreeformWindow' object found in the door and window section of the depository
here. If you turn the frame off, it will leave just the wallhole. You add new points to it by dragging any of the midpoint nodes. It's clever stuff!
Place the curtainwall in position in the wall then adjust the node points of the freeformwindow to snap to the corners.
Good luck!