No, although the parameters changed between yesterday and just now when I tried to do it. There is now a Z fare available on my existing flight that is only $40 higher than the fare I paid. I called and asked if I could buy up to F - initially they told me it would be over $800, then I mentioned that if I just went on to united.com I could get the same flight for $400. They were able to confirm this and then told me for that fare I would have to pay the $200 change fee in addition to the fare difference. I politely persisted that on the website I am usually able to buy up with no change fee but there was nothing being offered on the site - she said nothing could be done on her end and offered to transfer me to web support, so I figured why not...

Web support took a lot of time to look into it but had the same result. Buy up offer of ~$800 and only when I mentioned the fare available online were they were able to offer that but no waiver of the change fee. I said no thanks and they offered to check with a supervisor, but came back saying no waiver.

So I'm assuming that "buy up" is limited only to the fares that are the difference between what you paid and the full Y/B/M upgrades. And any of the discounted P/Z fares you have to pay the change fee?

If this weren't a corporate TA ticket I would pursue the refund/repurchase because of the schedule change, but unfortunately I can't do it in this case.