In a misconnect situation someone on an A class ticket can get any flight with F on it. If you don't like those options, you can get any other carrier with F seats for sale. If you don't like those options, you can take any UA flight that's full in F, take a Y seat, and gamble for the upgrade.

As for how it's prioritized, it's really a matter of how they rebooked it. If they downgraded you, you should be in Y. A GS on a Y fare as a displaced F passenger, should be at the top of the PR1 list.

The lists are so confusing and prone to error, you should make sure you're booked in Y, and the agent will put you on the PR1 before accepting the downgraded rebook. Many agents don't know these nuances (and the Helpdesk is often clueless as well), so the best bet is to assert yourself and suggest a more costly alternative to compel them to comply.

If they refuse to put you in Y and PR1, then say forget that then, book me in F on American (assuming they have space). That's a very costly rebook, so then they will be more likely to give you what you deserve on United.