When is your return? Which airline/routing are you using for the return?

I would print out the electronic ticket receipt that clearly shows a ticket issued in business class. I would then also use TTU to explain the situation - you want this IN WRITING and TTU will be in writing. Tell them clearly but briefly what happened and what you expect as a resolution.

If they absolutely can not get you into business class (the only reason IMHO would be the cabin is fully booked, not the business award class is no longer available), I would expect a refund of the difference of miles between Y and C and an additional 10K or more miles.

John