Generally, you're eligible to be rebooked on any eligible UA flight. However your situation is more complicated because of mileage and UA is not flying to ARN at all.

I would push them again to book a revenue seat on a Star Alliance partner (sounds like SAS could be an option at this point) and perform an "invol exchange" because of a market suspension schedule change.

This is the policy for your situation, when UA options exist:

"Same booking class. If original booking class unavailable, book lowest available (higher than original booking class, same cabin) up to and including M class in Economy, and same booking class for premium cabin."