Where (n) windows are going to replace (e) windows in their same openings, I create a class called "WNDO-EXIST OPG" and insert a size matched window object of the operation type "opening" into the wall. You can create a style that displays the opening as you like (showing wall linesor not, e.g.). This opening through the wall will show in plan and model whether or not you have the classes "WNDO-EXIST" and/or "WNDO-NEW" and/or "DEMO-WNDO" active (visible). For the window units themselves, I simply place them within the opening already created in the wall (make sure that Wall Insertion Mode is toggled off on the Selection Tool before you place them), and assign them whichever class is applicable. This gives me total control over correctly showing, for example, Demolition Plans, Existing Elevations v Proposed Elevations, all within one model file.