Originally booked on Delta V fare flying HKG-NRT-MSP-DTW-YOW.

HKG-NRT went mechanical on June 4 and Delta put me on UA flying HKG-ORD-YOW.

I believe Delta automatically open a case that offered me 7500 miles for the flight cancellation. (I never filed any complains).

I need some help on how to get miles for my original Delta flights.
Should I respond to the Delta case (I have a case number)?
Is 7500 miles a reasonable compensation from your experience?

thanks.