Looking for correct flashing detail for old windows with weight pockets. We have an 1927 cottage with original windows, exterior trim and shingles. Trying to figure out what to do when I reshingle. 

The windows are all in great shape and work well, so we’re keeping those with storms. When I redid the small garage/barn last summer, the exterior trim held the window frames in place. That was the only attachment point. Same with an exterior door I replaced last year. The exterior trim bridged the gap between the sheathing and the frame for the door/window.

That has me thinking I don’t want to remove the exterior window trim when I reshingle. But then I’m not sure how to handle the new housewrap and rainscreen if exterior trim isn’t removed. So question 1. Can you do new housewrap, rainscreen, shingles without removing exterior trim?

If the answer is definitively no and  I need to remove exterior window trim, then that leads me to: 2. Methods for keeping existing window frames square and in place when I remove the trim holding it in place , and 3. Flashing detail over the window pocket.