Yes, based on the response from American when I entered our Airline Booking #, I believe that is the case.
I can't believe there is a problem changing within the same seat class even if Viking paid for the ticket. Upgrading--yes, that I have to go back to Viking.
We are in economy to Miami, premium economy from Miami to Buenos Aires. We were in business class last year American DFW to Buenos Aires, food inedible, wine undrinkable (and flight attendant would not show label on wine bottle). So other than no lay flat seat, I will not be missing American business class.