I upgraded with Viking as it was slightly less expensive to do so. Also, I learned the hard way, if you do not insist that Viking ticket your flight, things can change.
Viking kept changing our flights making us have 2 connections and even took away our paid upgrade - frustrating since would happen about even 2 two weeks. Our original plans with air plus were only 1 connection if no round trip.
Finally got a great Viking air rep who told me that until they actually "ticket" the flights, Viking could change my flights and I might not get seats or even flight I preferred. Viking rep and I got everything settled as to exact flights we wanted, the upgrade we paid for, and the seats we specifically wanted. I was warned by rep that if they ticketed the flights, no changes could be made without high $$ penalties. I said do it, and we are happy.
From what I pieced together, Viking has enough designated seats with their contracted airlines that seems they can make a reservation for someone without actually paying for it - even though you, the passenger, have paid in full. To me this is sort of underhanded - like Viking is "floating" our fully paid money. Only when they tell airline to ticket the flight does Viking transfer your money to the airline and your flight is truly made as you and I would if we directly booked with airline. I may not be using proper terminology. but that's how I understood it to work.