I thought the idea of replacement windows was to leave the window frame in place, and install window units in the old frame, without disturbing the trim on the outside or the inside. Pella makes replacements which fit that way, can't say I enjoyed installing them, but at least you can do 6 or 8 in a day. My neighbor bought his from Sears, they took out the casement sashes out, and put double hung units in to replace them. One double unit they sawed out the center post and put a slider in. Course he told me later they cost 1,000 per window installed.