Let’s Encrypt now supports wildcard certificats, and there is an updated post on Atomic Writing called “Using a wildcard domain specification with a Let’s Encrypt SSL certificate” that goes over how to get a Let’s Encrypt wildcard certificate.
In January of 2018, Let’s Encrypt will begin supporting wildcard certificates. Until then, it’s still necessary to update a certificate file in order to add any new subdomains.
This blog post is largely intended as a note to self about how to update my Let’s Encrypt certificate for my personal website. But if you, like me, find yourself in the position of needing to expand your certificate to include another subdomain, hopefully you will find this post helpful.