How about a middle path? If the receptionist declines an upgrade with an obviously fake excuse, why not ask to speak with the Front Office Manager? If he does not budge, let him know that you will be staying elsewhere the next time - or even, cancelling the booking on the spot and moving (provided, of course, that you are still within the free cancellation period). In either case, you must be prepared to actually move elsewhere or not to return to that property.