Scottrick

Generally, anytime you want to do something like change seats or check in that applies specifically to the operating carrier, you need to do that on their web site. If you're at the airport and miss a connection, you should also probably go to UA if it was a UA flight or CO if it was a CO flight.

But if you need to make bigger changes to your itinerary in advance, like changing your destination, that might involve issuing a new ticket, then you should go back to the airline who sold it (CO) even if you're flying with someone else.