They all get donated to the "Save Ferris" fund. (the posts here are correct it becomes "ZERO" status and no longer has any use/value. You must cncl the space before midnight. You do not need to exchange it before midnight though. To not waste UA's space completely, and drive costs up <They will be passed on to the consumer> you should cncl as soon as you are sure though, and maybe someone else can save $$ buy buying your lower fare bucket instead of a) paying more, or b) flying on a non-econo plus carrier.)