I was not aware that there ever was a window upgrade package available for the Garfield. When I built my Garfield, I decided to change out all of the windows and doors to working ones. I took an inventory, did a lot of measuring and then ordered the windows through miniatures.com. I did have to cut out the openings and make adjustments so that the new working windows would fit. I even added dormer windows in the attic to allow more light in. All the windows/doors came unfinished so I stained them all. Many doors I did make from scratch and pin-hinged them as I could not quite match the doors available at miniature sites that would fit some of the smaller door openings. Many doors, however, I was able to order online and it was all worth the extra effort. If you look at my gallery you can see how I dealt with this.
Good luck on your Garfield build. It's truly a wonderful Victorian dollhouse!