How do I update the FileVault 2 recovery key stored in iCloud?

According to Apple's [documentation on Filevault 2] (https://support.apple.com/en-us/HT4790) (emphasis added by me)

Changing your recovery key

In the Security & Privacy system preference, under the FileVault tab, click "Turn Off FileVault" to disable FileVault. After FileVault is off, FileVault will begin to decrypt your drive. Once decryption is complete, you can click the "Turn On FileVault" button. Doing this allows you to enable unlock-capable users. You're also provided with a new recovery key and have the option of sending this new key to Apple. The old key sent to Apple will not be able to unlock your newly-encrypted disk. If you need to retrieve your recovery key from Apple, only the new one will be retrieved based on the Serial Number and Record Number displayed in the login window.

I think that this might be your answer.