Here is my answer =
1. the Bonus part (25% in my case) of the FCC will expire (12/31/2022 in my case)
2. the original FCC (what you actual paid but cancelled) will never expire, even after 12/31/2022, the set expiration date
A side note - I just booked a cruise last week, and I was able to use part of the FCC (12/31/22 expiration). To my surprise, the call center agent converted the rest of my FCC as OBC. I did not even ask her to do it because it is not doable based on my understanding (FCC can not be used for OBC or tax or fee; can only used for cruise fares).
Hope this helps.