Call. You're entitled to be rebooked in business class with no price increase. Suggest you research available options and know your preferred alternative before you call. Note that UA can only open award space on its own metal, so you will likely be limited to UA operated flights. If the agent resists opening award space on UA flights, politely end the call and call again.

It's a mistake to take it personally. All airlines, including UA, have been changing schedules frequently due to covid-related circumstances. It's not at all uncommon these days to go through two or three schedule changes for a flight booked far in advance.