So, if I am willing to pay the change fee (US$85 ?) I should be able to reroute via YYC or YYZ on the day of departure? Asking at YVR sounds like a good idea, but I'm worried that my already-checked baggage (if I end up checking) will prohibit a routing change.

Last time transiting through YVR - when I got angry looks at my US passport - was the first time I began wishing I were Canadian!