I was in the same situation a few months earlier and had a July 2020 Wind cruise cancelled by Silversea. I made enquiries to Silversea through my TA. I was, like you, keen to make another booking as soon as possible with their FCC offer.
While Silversea did not directly address the question, they drew my attention to the following terms and conditions (for 2021 sailings)-which is still on their website:
“The FCC can be moved voyage to voyage towards any other 2020 voyage”
“Once applied toward future cruise(s), the Cruise Credit will be rendered null and void”
“If you cancel the new booking say in Jan 2021 with reference from your example, it will be as per our standard Terms and Conditions as of now. You may refer Here for the latest update on our “Cruise with Confidence” policy.
My interpretation and that of my TA was that if you cancel the 2021 booking (or try to change to another sailing), the FCC is no longer valid because it is "null and void" and you would likely lose all your money, meaning the cruise you booked becomes binding and non-refundable. There is not even mention of getting another FCC back because it is not movable voyage to voyage.
The alternative is if you are absolutely certain that you will sail on Silversea in the next two years you can get the 125% (or 110%) credit and use $1,000 of the FCC to pay the deposit. I have confirmed with Silversea that part of the FCC can be used as a deposit, and you also get all future cruise / early booking bonus.
"For Future Cruise Credits received as a result of recent cruise cancellations, it will be accepted as the deposit (minimum US1,000) for the Double Bonus promotion"
You can keep the rest of the FCC to see if the situation will improve, and only make final payments with the FCC at the last possible moment. If you cancel the booked cruise you will lose $1,000 pp instead of the entire FCC.
I am sure if Silversea cancels the cruise some kind of arrangement will be made.
I decided (reluctantly) to take the cash refund.
Please let me know if this is incorrect or there are new developments.