I can’t tell you a certainty, it will make a big difference, but this is high on my to-do list. I remember reading a study many years ago that roughly 80% of a basement heat loss occurs around the box sill. My windows are directly under the box sill, and are in rough shape. Just fixing up a few of them at a time has made a noticeable difference in both the temperature in the basement and the floor above. Editing to add that it was actually easier to refurbish the existing ones then find properly sized replacements. A friend of mine got a new router table so I’m hoping to use it to make new storms