I originally made my mdr reservations at each of the four to try them out, but not all had my chosen time of 6pm, but later ones so I selected them.
I checked availability a week later and found that the 6pm’s were now available and switched them. You must first cancel your first one and then you can reserve the new time.
You, of course, can stop in anytime at your discretion and most likely can be seated within minutes.