I am assuming your trip is international. If so, and you have domestic segments on your trip, then you can have the SWU applied to those segments, if upgrades are available for those segments. Applying the SWUs to the domestic segments will keep SWU "alive" for the international segments past their expire date. My experience is that the AA phone reps will not applied the SWU to the domestic segments, or even suggest it, unless the international segments have SWU upgrade available. This is because most travelers don't want to waste SWUs on just the domestic segments if the international segment upgrade doesn't clear. But, in your case, you are at a use them or lose them situation, so why not?