I just checked in online for my SEA-IAD flight tomorrow morning, for which I had a confirmed upgrade to C using miles. However, when I checked in, my only options to support it were to use 500 milers, a CR1, or an SWU.

Given that I have a bunch of 500 milers, I used them...but isn't this technically not allowed? Typically changing to 500s requires going back on the waitlist, and C appears to be full. I'm not complaining, mind you, but I found it curious.