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If you buy the upgrade certs from your friend and United is able to determine that you did so, your upgrade chances are zero, as the Terms and Conditions of the certificates state that such a transaction will void them.

The waitlist for upgrades supported by a SWU is cleared by order of when you were added to the list, first on, first off. Shortly before the flight, the list is re-sorted by Frequent Flyer status. So, if you haven't cleared by the time the list is re-sorted, unless you have very high UA status, your chances are not good.

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