Unfortunately, this is correct. If you had booked the flight using a UA flight number, you could have upgraded the flight using UA miles. But you cannot use CO miles to upgrade a UA flight and you cannot use UA miles to upgrade a flight booked with a CO flight number, even though it is UA metal.

From the airline's point of of view, the economics of a direct sell vs a codeshare sell are quite different - hence, different rules about using miles on a codeshare flight.


To TimeZone - a codeshare flight is one in which one airline buys seats on another airline's flight and puts its own flight number on that flight. In this case, CO purchased seats on a UA flight and resold those seats with its own flight number. Thus, the same flight has both a UA flight number and a CO flight number and one can purchase a ticket from either airline using either flight number.

This is done for numerous reasons but basically it comes down to a joint marketing effort between the airlines that theoretically gives more flexibility to the passenger and thus makes these flights more "sell-able" for the airlines.