Insulating a replacement insert window
A fair amount of the heat loss from a window is through the frame and mullions.  If you are going to replace windows, normally you would replace the frames too.  This not only gives you better performance overall, but it's usually easier.

When you make a custom window at the factory you have a whole set of jigs that you can use to cut, hold pieces, etc.  It's a fairly fast operation, although the machinery doesn't come in crackerjack boxes.

If the windows are traditional wood, you could remove the putty, remove the old glazing, replace each each a bare sealed unit, and re-putty them in.  This isn't affordable to hire trades people for, but if you are accurate with a tape measure, and have more time than money you can get somewhere around half or 3/4 of the performance increase using your existing frames.

Also try a search for "Building your own windows and doors"



In passing:  Right now energy is cheap.  The return on investment on new windows is often negative -- they never pay for themselves.  I have a 35 year old house.  We've replaced 2 sets of windows.  One set when a bird flew into it and shattered the outer pane.  The windows were custom shapes, one a pentagon, and two trapezoids, and on the second floor.  Since all had broken seals and were grungy inside, we opted to replace all 3 while the lift was here.

The second set was to replace all the aluminum sliders in the house.  These were more than radiation leaks.  An aluminum slider in a cold climate is a continuous source of drafts and moisture.  (Water condenses on the cold aluminum, then drips onto the wall/into the framing.

The new windows that replaced the sliders were made to be slightly smaller than the rough openings.  They were able to remove the brick mold from the outside without damaging our vinyl siding.  The replacement windows had a molding build in, slightly wider than standard brick molding.  This covered the joint.  From the inside, foam insulation was sprayed into the space to seal it.

So it may well be possible to replace the windows without having any stucco repair.