Join Date: May 2010
Location: PHL
Programs: AA EXP, UA *S, Hilton Diamond, Marriott Titanium, Hyatt Exp, IHG Plat, National EE, Sixt Plat
Posts: 648
My understanding is that for the agent helping you with the change, (re)adding you to the upgrade list is a manual step they have to do once the new flight is confirmed. If you're working with a good experienced agent (phone, lounge, etc.) they'll often remember to do it for you. But I'll always ask them just in case, because it's a step they can easily forget otherwise.
If you're dealing with a particularly surly agent, especially if handling this at a busy gate shortly before boarding, they'll sometimes say "you have no chance, there are already xx people on the list" as if they can't be bothered to click a few extra buttons. When I insist, as an EXP, I usually end up #1-3 on the list, and often clear moments later. You're always entitled to be added to the list, even if the agent claims there would be no point. If you have lounge access, try to handle it there instead of the gate whenever possible.
On rare occasions, and usually only in the worst IRROPS, I do sometimes get rebooked straight into F if my upgrade on the original flight had already cleared. At this point, I just go by myself a lottery ticket and call it a day.