You must generate a new CSR through GeoTrust. You can do this through the link below:
https://products.geotrust.com/geocenter/reissuance/reissue.do
To qualify for reissuance, all existing core certificate details must remain the same (including fully qualified domain name)Also, keep in mind that the certificate listed in orbit will no longer be valid, the reissued certificate will only be emailed to them.
More information:
http://www.geotrust.com/resources/refund/index.htm