First, my sincerest appreciation for all the replies. A special thanks to Snoogie for providing the explanation and reassurance. Nobody wants to have their new car taken apart and re-assembled, especially electrical components. This particular dealer has a very good reputation for service, and like other dealers, I am sure has access to an online service tech database, if needed. The source of my initial concern was that the service advisor originally told me that the combiner (only) can be replaced; turns out the entire unit has to be exchanged out. From the PDF, it appears that a critical step is getting the main wiring harness back together correctly. I guess when I heard what has to be done, I started having flashbacks to ten years ago when I owned a new Honda Pilot. I had the local Honda dealer install the Honda accessory cassette tape deck into the dash (replacing a useless tray). This involved taking the dash plus the center console apart, which is why I would not even attempt it. When I got the car back, there were scratches and scrapes on the dash and console trim pieces, including the gear shift knob, which looked like it was removed via a vice-grip pliers. I trust this dealer will take extra precautions to prevent anything like that from happening.
As to why the AD needs to be replaced: long story-short is it was not the car's fault. The unit does still work. Appears that when a car is shipped from the port to the dealer there is protective clear wrap covering the various instrument components. (Snoogie - or anyone - please feel free to comment if that is not correct). A piece also covers the back of the (closed) AD. Seems that when the protective strip was removed from the back of the AD, some adhesive residue was left on the unit (combiner). When I took delivery in the beginning of September, it was a bright sunny 90 degree day, and the combiner looked okay on inspection. However over the past 5 weeks I began to slowly notice several streaks looking like accumulated dust sticking to the back of the combiner across the entire back of the unit. Rather than fooling around with it with some cleaning chemical, I called the dealer's service dept and asked if they could clean it off for me as I felt (rightly or wrongly) that that (adhesive residue) should have been checked for prior to delivery. Ten days ago, I took it in and whatever someone did, it now looks ten times worse than before. That is when the service advisor stated the dealer would replace it with a new unit. I guess Mazda is lenient with warranty reimbursements. I'll try to put up a picture here so all can see what I am talking about.
Again, my thanks to everyone here. Great forum.