Welcome to FT, the_fox ^

If the fare's still showing up on Orbitz, it's likely still available on UA. As you will find out if you spend much time in this forum, the ual.com website is likely considered to be the absolute worst airline website around, so the fare may exist but may not be bookable for a few hours or a day or two because of problems with the site. I'd suggest that you also call UA and ask if that fare is still available. If you're told that it still is, then it's likely to show up again on the website. Your other option is to book it on-line at the higher fare and then "refare" your itinerary when the fare is reduced - this will get you a refare voucher for the difference (do this by phone and NOT online) that you can use at another time, although you can't use the voucher to make another online booking.

Good luck!