Originally Posted by
eigenvector
On the other hand, you'd have a tough time characterizing it as theft if it's sanctioned by the seller's representatives. And very few FAs would deny a pax from moving to a vacant, non-premium (bulkhead, etc.) seat in the same cabin.
That depends entirely on the airline and it's policies. Some airlines DO enforce the revenue profile of the cabin, and are willing to sell an upgrade in-flight. Others do not, and still others have a rule but the FA has the final call.