Switch from Fish Buffet to a Platinum Rank of a lower level/rank:
All Octopus, Crab and Shrimp can switch to Platinum Fish
All Whales can switch to Platinum Octopus
Switch from Platinum back to Fish Buffet:
You can always switch back to regular Fish Buffet to try for a higher level
Platinum Fish will start at Bronze Shrimp
Platinum Octopus will start at Bronze Whale 3
You will not be able to switch back from Fish Buffet to Platinum Rank for a period of time equivalent to the time limit of the new Bronze rank. Refer to the Fish Buffet Ranks for the time limit of each rank here or by going to Settings > Rewards Program > Fish Buffet Ranks.
Eg: If you switch from Platinum Fish to Bronze Shrimp, you will not be able to return to Platinum Fish for 15 days.