I need to re-sign a macOS app I have already built and signed. Is this possible? I need to be able to sign it with my client's certificates rather than my employers.
This is done with codesign. On their system, assuming their certificate is in their keychain, then can use codesign -f -s <identity> <path>. The -f option indicates that previous signatures should be replaced.
codesign -f -s - --timestamp=none --all-architectures --deep /path/to.app