A customer wanted new windows to replace old single pane windows. He bought the windows and helped me put them in! We came up with a system that made the installation easy. Without removing the siding, a space for the trim must be cut exactly so the new trim can go in tightly and neatly. It was a two step process.
First we depth cut around the old window brickmold trim with the 1 1/2″ spacing provided by the narrow side of a skillsaw shoe. This had nothing to do with removing the existing window but removed just enough siding to install the new window’s flange using the created space. Once the new window was in we used a jig, secured against the new window, to depth cut a 3 1/2″ space for the new trim. It worked great. Check out the process in the photos below.