You can NEVER divulge client info to a third party without your client's written authorization.  Hell, you are not supposed to even acknowledge a professional client relationship exists when a mortgage lender calls for info without prior authorization from the client.

Your E&O insurance likely has the boilerplate language to use; so does the AICPA.  Circular 230 may even have the specifics.

You don't 'own' your clients or their info.  What you are selling is the possibility the clients will continue to use the new owner for tax prep.   A letter of introduction to the new person (when you ask for permission to provide info to that person) would be a good idea.