Colleague of mine asked me whether CO sells upgrades at check-in for BusinessFirst. He is flying from a domestic station to EWR on to Europe. If such upgrades do exist but he isn't offered one at check-in, can he check at EWR to see whether one is available for the TATL leg, and if so, with whom ?

Also, what's the best option to buy an upgrade when checking-in ? Checking-in at the airport or online as early as possible ?