If you already have a multi-domain SSL Certificate, you can simply start the re-issue process and add the corresponding number of SANs to your Certificate. The regular checkout flow will prompt you to purchase additional domains.

The additional domain names can be changed during reissue as well, however there are several features that should not be forgotten:

• Comodo multi-domain certificates allow to change the SANs completely, e.g. from domain.com to example.net.
• in Symantec/GeoTrust/Thawte certificates it is only possible to change the subdomain of SAN domains, for example from domain.com to mail.domain.com. The option to use the totally new domain names instead of old ones in the SAN fields is not supported for the certificates issued by the brands mentioned above.

The primary domain of the multi-domain certificates cannot be changed during reissue. The only solution is to request cancellation and refund of the current certificate, purchase the new one and activate it from scratch using the CSR with the new Common name.