you may get lucky. Point out the wide open C on the route you want and acknowledge up front that it is not quite kosher - honesty goes a long way as so many people are out to scam the system and are too outrageous for words. Actually this should suit UA to get you out of a potentially oversold flight as it could well save them real live cash if they have to bump someone or are able to sell a full Y ticket at the last minute - and it is a cheap way for them to burn up your SWU since it costs them basically nothing if the flight is wide open. Whether the agent is into activity based accounting and feels empowered to do this is another matter. Trying to do it before is a waste of time - the whole world would be changing their itineraries for spurious reasons if they folded to this.