Changing a cert’s domains, include all domains that you want to have in the cert (also subdomains):

sudo letsencrypt certificates

sudo letsencrypt certonly --cert-name name from first command -d first domain of existing cert -d second domain of existing cert -d and so on -d first new domain -d second new domain --dry-run

When all runs fine, do it:

sudo letsencrypt certonly --cert-name name from first command -d first domain of existing cert -d second domain of existing cert -d and so on -d first new domain -d second new domain