I have completed one retrofit window by removing the glass, then using a Sawzall and various tools to cut away the middle steel piece and the hinge. It was very difficult to get the Sawzall in the tight area for the hinge. This is how Home Depot said to do it. Due to the limited space between the hinge and the window ledge, it took many hours to get it out. Then the old steel frame was so wide, the retrofit window frame was just short of covering the wood trim on the outside of the house, leaving the window looking a bit unprofessional. On the inside, the window was so far from the edge, I had to use 4 inch nails. It just doesn't seem to be the best way, despite the good intentions Home Depot had when they recommended it.
Tomorrow I will likely try a replacement window for the next one. I have hired a general contractor to work with me and teach me what he knows. This obviously will require chipping away the stucco, etc. so it may be more difficult than dealing with the retrofit. However, based on how it fits and looks,I don't recommend the retrofit. Good luck to you.