Slim.

VS don't really do Op-Ups. And they get plenty of people trying for upgrades because of being on a honeymoon etc. I am not saying don't try... but don't expect anything more than what class you booked. If you are travelling in uniform and get that one check in lady who's into a military man, then maybe you'll get lucky

Best bet is to try getting an upgrade at check in. Ask straight away what the upgrade prices are, they are generally cheaper at the airport. Calling in advance involves a re-ticket and is almost always more expensive (but you can check that out too).