Most UA/US 2 class domestic are First/Economy redemption for other *A carrier. This is because UA/US pegs the business class into O Class (first) instead of I Class (business)

The only exceptions are routes interlined with an international service and existing CO 2 class service.

If AP already confirmed you business class on UA which got changed to a 2 class, you should be grandfathered and protected to First. That happened to me too. If you are looking for seat assignment, call UA directly.