On AA, companion upgrades can be reserved in advance, and the companion's upgrade priority assumes the level of the person upgrading the companion. Companion is charged the same number of stickers as the upgrader, and these come out of the upgrader's account. I know this is true for Exec Platinum (equiv to Chairman's Preferred) and, I believe, Platinum members, but am not sure about AA Golds.
On NW, companion upgrades are permissable at the gate (within 2 hours of flight time) for Platinums and, I believe, Golds. But the companion's upgrade priority is below other Elite members; the companion does not inherit the upgrader's status. I believe CO is the same.
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/globaldude