or top them up with water + concentrate made to the same gravity as my original must.
I guess we really didn't answer your entire question. I wouldn't add more concentrate at this point. Your primary fermentation is already finished and by adding more concentrate you'll likely restart an active aggressive primary fermentation any you'll be back at step one. When I make wine I always make more than what my secondaries will hold so I can make sure they're topped off when I rack.