I had pretty good luck on my last trip -- reserved on united.com (with the correct fare class, believe it or not), called 1P reservations to get my upgrade, purcahsed while on the phone, then the 1P rep confirmed my upgrade right away. Worked pretty smoothly, thanks to FTer's who provided the suggestions to do it this way.

As I mentioned in my previous post, it's frustrating, but I've found that if I start an iteinerary from scratch, I usually get the correct upgradeable fare classes that I ask for.