When an X.509 certificate has to be renewed it is helpful to use
the old PKCS#10 certificate request as a template, so that the
distinguishedName (DN), the subjectAlternativeName (SAN) and
a certificate profile name don't have to be typed-in again.
The old public key in the existing certreq is replaced with the
new key and the signature is re-generated using the new private key.