Every aspect of how to install replacement windows in every different situation is a given. The common denominator will always be do the best possible job you can.
After removing all the pertinent parts and the dimensions are accurate a dry fit has to take place, what we look for is any abnormalities, something that can interfere with our progress going forward.
This one house comes to mind with aluminum windows preceding our new double paned white vinyl replacements. The old window frames were not the issue though. The problem occurred with the existing stucco being too thick and not allowing our new vinyls to seat far enough inwards to get any fastening bite. Problem solved, the decision went for stucco removal. Post dry fit marking the stucco on the outside edge of the new frame and with a 4″ DeWalt grinder using a diamond blade, just scoring the stucco surface. Taking up to 3/4″ in areas applicable until the proper fit had been achieved to install the new windows and without any exterior patching or painting involved, Booooooya!