Some time ago I changed the domain of my blog - from www.kadder.de to www.techblogger.net. There was a pitfall with the use of Let's encrypt.
The approach: when the domain is changed, of course, it should be forwarded from the old domain to the new one. The problem: if the certificate of the old domain expires, SSL calls lead to a certificate error. Rankings and visitors who previously called the old site will be lost.
After some experiments with the Nginx forwarding rules (to prevent the .well-knownwhich is used for the webroot certification of Let's encrypt is needed is redirected to the new domain) the solution is obvious.
In order to continue to use the automatic update of the certificate (a very good Instructions for which there is Digitalocean) one approach is to extend the existing certificate for the new domain. This must also include the existing domain.
Once this is done, the same certificate is configured for the old domain as for the new one, with the result that the redirections will work again for SSL connections.