Yes. On the Mamiya 645 Super there is a small black release button next to the lower-right screw on the back/insert area. Pressing that tiny button releases the dark slide. It’s easy to mistake the larger button for the release, but the correct control is the small black one that looks almost like a screw. Once you use that release, you can free the dark slide and then remove or swap the film back as intended. If there is film loaded, be careful: removing a back or slide improperly can expose the film.