If I encrypt my HD using FireVault, what happens to my (previously unencrypted) TimeMachine backups?
Your backups don't get encrypted unless you explicitly convert or set them up. Time Machine will probably warn you when you have an encrypted source and a non-encrypted destination, but you can choose whether the source or destination or both are encrypted or non-encrypted to suit your needs.

Apple's FileVault and encryption is reversible so you can keep your history and go forward and back (assuming you don't mind the delay to process every block on the storage each time you convert or revert the encryption status)