It will escape. I think I would try and corall the down as much as possible with permanent and temporary stitching, before opening up. When I did open up I would move slowly and use bags made of paper as temporary down holders not plastic, because of static electricity. You need to be in a small space with no draughts. If you do not need a sewing machine you could see if you could do it in a tent inner erected inside. You can then pick up what escapes.