I have upgraded using CO miles on the VS codeshares a number of times. It is indeed rather painful and involves many calls to CO, visits to the CTO, and a couple of bottles of single malt to calm me down afterwards. They have to issue paper tickets together with electronic mile deductions. Woe is you if you attempt to make a change once the ticket has been issued. The process has to be restarted and the tickets have to be reissued.

I'd say of all the transactions I have done with CO agents over the years (and we are talking multiple years of 100K+ flown miles), this is the one that really stretches their ability and knowledge, and wears my patience ragged. Mistakes are made EVERY time (to be fair, sometimes in my favor, in which case I keep my mouth shut) and it is never a stress-free transaction.

In my day job, I work with some fairly complicated material requiring minute attention to detail and a lot of patience, and dealing with CO on the VS codeshare upgrade is always memorable. I can't imagine someone who's not anal and ready to spoil for a catfight with a complicated and elusive adversary not being reduced to tears in frustration over such a transaction.

As for availability, I have been reasonably fortunate. I usually make my arrangements two weeks before the flight, and give the agent a couple of days window around which to fly. Fares can vary all over the place. If you are game, there is a lot more "flexibility" around Y fares than around H or K, but you'll pay for the privilege. On my last upgrade game, I bought my tickets about 2 weeks before the flight, a BOS-LHR-SFO-BOS for $1,900 or so. What really ticked me off is that the BOS-LHR-SFO was VS UC ticketed in K, and cost me about $700+miles. The rest was for a full Y SFO-BOS! I also priced the same route for the SAME DAY but via EWR (on the codeshare) and it was coming out at almost $4,000. So be patient, flexible, and steel yourself for some tough negotiating...

PS It was worth it. I love VS UC. See my trip report in the Trip Reports if interested.