I always wondered about that. Is there some rule or form that the superdelegates fill out that requires them to vote for a stated choice, or is that still optional?
They all have the option to change their minds up to the moment they cast their votes. In some cases local laws may require them to vote for the winner of their district/state.
Until they vote, Clinton has not clinched this nomination. Not now, not after California. She will need a couple of hundred superdelegates to put her over the top at the convention.