Hi Colin,
Enhanced rate PIP entitles you to everything that high rate DLA did. As you say, this is not a change of circumstances.
It might be worth contacting the CA and Council Tax people, just on the off-chance that they have been notified that your DLA is ending but not that you are now entitled to PIP! But this is really a 'belt and braces' approach . . .
Nothing on this board constitutes legal advice - always consult a professional about specific problems