Originally Posted by
sdm1130
CO does not use any upgrade instruments so your options are to use miles (domestic) or miles + a copay (Business First markets).
For a non-elite there is also a co-pay required for domestic upgrades from cheap flights. This is a recent change that allows non-elites to upgrade from those fares; prior to the co-pay change on those fares non-elites were just not allowed to upgrade on domestic flights with cheap fares.
To the OP, if you get the miles and call in for the upgrade and the inventory isn't immediately available, ask to be put on the waitlist for the upgrade. Much easier than calling back and checking all the time. And you generally have to ask, as the agents will not always offer it to you.