Well, EU regulation EU261/2004 means that the operating carrier is legally responsible to rebook you on the next comparable flight. Roughly speaking, this should mean they put you on the fastest connection to YYY which has availability in business class.

That regulation is costing EU airlines serious money. They sometimes fail to follow the rules, particularly if you're not firm on your passenger rights.

Had you been downgraded, you would have been eligible for compensation. The booking class "K" makes no sense once you were properly rebooked on the next day's flight in C.