Well, now that the certificate has been revoked, it's too late for the FBI to do anything: either users saw that the certificate has been revoked, or they didn'

Well, now that the certificate has been revoked, it's too late for the FBI to do anything: either users saw that the certificate has been revoked, or they didn't yet but if a new certificate is installed they will see that it's a new one with a different fingerprint (at least their browser should warn them of that).