It is no problem at all to shorten a raincoat with plain sleeves. But, if the coat has sleeve straps (going through loops) and will needs considerable shortening, you might have difficulties moving the loops further back. It is very likely that the previous stitching will leave marks in the cotton material. I've just checked with a Burberry trench coat, the loops are about 2 Â½" away from the bottom edge. You can easily shorten by Â¾"; more will be problematic as the straps will be a bit close to the edge. If the coat has sleeve tabs, caught in the seam, it should be no problem to move those further back. Don't forget, this is a raincoat. You don't want the sleeves too short. They should cover the jacket sleeves as well as the shirtsleeves.