Hi Slow and Silly !
Pretty much as these guys have said really - depends on the booked figure of course, if there is space available and the meals are there it just depends on who your gate team are and who the dispatcher is !
It also makes a difference which airline staff you are, ie: UA and BD will always try and upgrade each other as we know it's reciprochal ! SQ on the other hand, you haven't got a pork chops chance in a synagogue of an upgrade - bloody good service tho !
Basically keep yourself pretty smart, be a happy chap checking in and always politley ask if you can scrounge a seat further forward in the cabin !! It is always nice turning left instead of right from the jetty onto the plane !
rgds Dispatcher
