AA and UA have been known to extend one or two SWU each year by exception for about 3 months. One certainly cannot count on it!

CO - just started giving 4 upgrade certs this year - no fare restrictions so I doubt many will expire. CO also allows them to be used for other people making them easier to use.

DL - 75K and above flyers can earn upgrade certs. Upgrade certs are only valid on full fare (M, B, Y) for intertnaional tickets so most expire unused as most flyers cannot buy unrestricted tickets. DL upgrade certs can also only be used if the person who earned them is flying. This year DL for the first time is offering flyers other options instead of upgrade certs instead of them - flyers who are educted/understand how hard they are to use will probable not earn upgrade certs with DL now unless they fly very high fares normally.

UA - 100K flyers earn 6 SWU. UA SWUs can be used by anyone so that halps. Many SWU will go unused because a good number of the people who earn them do not normally fly interntionally. More will go unused because some peoople don't normally buy W and above fare class tickets when they do fly interntional and SWU are only good on W and above fare. More SWU will expire because some routes seem to sell out in business and upgrades often do not clear. My guess is only about 45% of SWU will get used this year.

AA - 100K flyers earn 8 SWU. Some will expired because flyers don't fly interntional. AA SWU can be used by anyone and ALL AA fares qualify, making these the easiest to use upgrades (tied with CO). As all fares qualify and anyone can use, I think few will expire.

US - 100K flyers earn 2 US SWU. Person who earned them has to by flying to use them so they will expire if the person does not fly interntional. As few are earned, fewer will expire.

The smaller US ailrines don't have interntional upgrades.