Add additional subdomains to a letsencrypt certificate

taken from: http://stackoverflow.com/questions/38302401/letsencrypt-add-domain-to-existing-certificate

I used the following command originally to register some certificates:

/opt/certbot/certbot-auto certonly --webroot --agree-tos -w /srv/www/letsencrypt/ \
--email me@example.com \
--expand -d example.com,www.example.com

… and just now I successfully used the following command to expand my registration to include a new subdomain as a SAN:

/opt/certbot/certbot-auto certonly --webroot --agree-tos -w /srv/www/letsencrypt/ \
--expand -d example.com,www.example.com,click.example.com