Glasses focus the light into your eye, still relying on your eye to focus on distance. The HUD in my example would flatten the distance and show a 2D picture in front of your eye at a point where you can naturally focus.

Like I said, I don't know if this is possible.

Yeah, seems like a more salient benefit to the visually impaired from a HUD would be for a patient with significant damage to regions of their visual field. Then the computations available with the wearable could be used to apply a real-time transformation to the video so as to push more of the video stream's information into the regions of their visual field which still work.