After the certificate is issued, the number of available SANs (hostname slots) cannot be decreased. You can, however, change the used hostnames via reissue.
Another solution is to request cancellation and refund of the existing certificate within the acceptable refund grace period (15 days after the first issuance within 90 days after the purchase date) and purchase the new one with the correct amount of additional domains.
This means that:
- The SAN, added to the Multi-Domain certificate any time after its purchase, will cost the same price as the SAN ordered along with the Multi-Domain certificate during the initial purchase;
- The SAN will expire together with the certificate it is attached to regardless of the exact date of SAN’s purchase.