Is it possible to decrease the number of SANs after the SSL Certificate has been issued?
No, the number of SANs cannot be decreased. You can, however, change SAN domains or subdomains by re-issuing the Certificate.
You can, however, request cancellation and refund of the existing Certificate, purchase the needed SSL Certificate, and activate with the appropriate number of SANs.
If a SAN add-on was purchased, but was not activated it can be cancelled and refunded any time unless it is activated.