There is a difference between CO, and for example, AA. You cannot game the system at CO by making calls for upgrades, they don't take calls for upgrades. I am a Plat, have seen empty inventory online and they will not upgrade. The response is that you will get an upgrade if the computer gives you one. You cannot rerequest the upgrade at certain point before departure and jump the line, as can happen with AA. (There are plenty of threads on AA regarding how it is done.)

About the only thing you can do is checkin online and that will give you priority over other Silvers with the same or lower fare class. When you do the online checkin, you will be asked if you want to seek an upgrade, if you say yes and do not clear, your boarding pass will note that you have been added to the wait list. Upgrades are processed first to full Y fares, then to Platinums by fare class, Golds by fare class and Silvers by fare classs. As the poster above noted, Silvers do not frequently obtain upgrades.