Welcome to the Disboards
Great first question/post. I don't recall ever reading this question.
I can only hope you didn't read this whole thread looking for the answer.
Are you staying concierge level? If so I'm sure they will work something out. Like trade the SB for FB.
If not ... don't know. Maybe they will charge you for 8 at FB, 4 at SB, and then you promise not to have more than 6 adults at a time on FB. That's my solution.
You could ask folks on your Dismeet if someone is willing to trade a FB for a SB.
I can assure you someone on FB would rather have a SB cabana. How do you find them?
Make an announcement onboard over the intercom.

JK