If you want to switch from using a regulair certificate/certificates to using Let's Encrypt's new wildcard cert there are some things you still neeed to do, here is a simple command that should suffice:
sudo certbot certonly --cert-name YOURCERTNAME.com -d *.DOMAIN.com,DOMAIN.com --server https://acme-v02.api.letsencrypt.org/directory --manual --preferred-challenges=dns
Certbot will walk you trough the verfication using DNS which currently is the only option with wildcard support, you'll need to add two TXT record for verification.
Good luck!