Using CO Website to Upgrade
 

CAn someone tell me the process of using the CO website where it gives you the option to upgrade an individual leg of a trip or the whole trip? I see where you can click on which option you want then below is the button to either Upgrade or Cancel. If you click on the upgrade, at what point does it release your old seat? I've never used this before and worry if I click on the Upgrade button that it would release my old seat and if I don't go through with it I would eb screwed. So how far can you go while everything still stays the same?