alfred, we're in the same "boat." My husband's a teacher, so we have to vacation during the peak season.
In any case, we were in exactly the same situation on a HAL cruise we took about a year-and-a-half ago. We were confirmed for our preferred fixed dining time but, when my niece and her husband booked a couple of weeks later for the same cruise, they were told that the only availability was in open seating. As Cruise Fanatic mentioned above, we cross-referenced our reservations and planned to talk with the maitre d' after we boarded. It wasn't necessary. Our niece and her family ended up being assigned to our table without us having to do a thing.
I still suggest, just in case, that you plan to make the maitre d' your first stop when he's available for change requests (do not wait until dinner time). You can acknowledge his assistance at the end of the cruise.