ANSWER:

Yes, they will revoke the old certificate.

The way that SSL revocations works is that, inside the certificate, the vendor places a URL where the client (e.g. browser) should check if the certificate is still valid (called a CRL).

So there is no hard and fast answer to this, it depends on the client. In some cases, like this article, it suggests it won’t be checked at all.

by Jay Shah from http://serverfault.com/questions/397744