A sweeping revocation wouldn't work, as you can only revoke certifcates that have not been tampered with. You must issue a SHA-1 certificate to everyone and then change every piece of software that uses certificates not to trust certificates with an MD5 hash in the trust chain. If someone else has successfully performed this attack, he might posses a CA certificate that is valid until 2038 and you have no way to know about it until he uses it against you.