That the 7753 in gen Navitimers is modified simply is not true.
On a gen Navitimer you have to change the date by running the time through several days until you reach the current date, so no quick set.
The only thing the rep factories would have to do is a new dial and date wheel. The old 6-9-12 case is a straight fit as you already mentioned. The A7753 will work in this combo just like the gen does, too.