As others have suggested, you're probably better off booking with HP, but ensure your US # is tied to the rez and they are aware of your CP status. While your upgrade still may not clear at the window, my experiences lean toward this being the most effective way to boost upgrade chances. And if you don't clear at the window, you can do what the rest of us do, and that is call HP and inquire about upgrade availability. US may tell you they can't "force" the upgrade, even though, as some have suggested, the CP desk now has this capability.

I caution you that my upgrades have never automatically cleared on HP regardless of how the ticket was booked. I have had to call every time. Some here have had better luck than me, however.

Hope this is helpful.