Update to my own question....
I took my 2015 to another Ford Dealership and was told (by a Service Adviser) that the truck would have to be returned to stock prior to changing the tank parameter. They also said it would cost $100.00 and I'd have to leave the truck with them for a day.
Since it was suggested here on the Forum that anyone with an IDS for Ford's (along with the knowledge to operate the system) can do this, I went to an Independent Shop, owned by a guy who was the Service Manager/Shop Foreman for a Ford Dealership for 35 years (before getting fed up with Corporate America). He said that the programming for the tank calibration is through a different module and has ZERO affect on the tuning. As we were talking, he grabbed his laptop computer, walked out to my truck and changed the parameter in less than 5 minutes while we chatted about the good old days. He refused to take any money, just asking to send some business his way. My digital display is now showing 815 miles to empty!
Long story, even longer: You don't have to return your truck to "stock" tuning prior to recalibrating the parameter for the fuel tank.
The only negative to the Titan 50 gallon tank is the first fill-up. It cost me $160.00! Thank goodness diesel was $3.19 per gallon. The last tank I installed was on a 2010. It was a 52 gallon tank and diesel was $4.39 per gallon. That was a $225.00 fill-up!!!