How can I allow a string of multiple signers and keep existing data from being changed and how can I keep a signed PDF from being saved as and changed with the Sigantures saved as form?

I am having difficulty figuring out how to allow only additional signatures to a form once it is signed by the first person in a string of people who will sign it. Is there a "signing existing signature fields" in the "changes allowed" permissions? It does not seem to be an option. We are signing forms and can easily save them as and change data in the form while still showing the existing signatures.

Thank you,

Chris Carter

U.S. Department of Agriculture

Albany, California