It's 100% normal for the airline to not provide any refund of the fare difference (in $ or miles) until you've actually flown the leg in coach. Lots can happen to get you back up to FC. Certainly reach back out to them after your trip.
And frankly I'm surprised you got the $200 "compensation" voucher before actually flying in the lower class of service, although I guess you still have to deal with the hassle and uncertainty.