From what I have seen , CanadianFlyer's assesment is correct.

When calling for an upgrade, if an F seat is available when you ask AND you are eligible for an upgrade, the seat is yours. I have upgraded one half of an itinerary many times.

I don't think they waitlist anybody over the phone. The waitlist starts at the airport 2 hours before flight time.

I you are using a OnePass reward for the upgrade, you have to ask yourself if the upgrade for One segment is worth 10,000 miles. If you are upgading due to elite status, then get the upgrade as soon as possible.

Either way, keep calling for the other segment. I can't begin to tell you how many times I have been told no the first time I have called and yes, the second or third time.