If you have lost your certificate, need to move servers or had a problem with the installation then it may be possible to re-issue your certificate at no extra charge providing the common name remains exactly the same.
Email Authentication is available as an option to verify ownership of your domain.
How to complete DNS authentication with Comodo orders. It is possible to add a DNS CNAME record to your domain in order to validate ownership when placing orders for Comodo products.
Information on how to reissue a Comodo SSL certificate.