First here is what I did on Ubuntu 16/nginx Digital Ocean droplet:
Installed free SSL cert as per their guide for old domain OLD_domain.com.au, all good.
Domain changed to NEW_domain.com.au.
Removed old cert and insta...
I fixed this:
verify no other conflic files grep -ErI 'olddomain.com.au|newdomain.com.au' /etc/nginx/
backed up conf files
sudo nginx -t
sudo service nginx restart