Please note that the bncert tool uses the Let's Encrypt service in order to generate an SSL certificate. In order for this to work, Let's Encrypt needs to validate that the domain the certificate is being issued for is actually resolved to the machine where the request was originated from.
Taking that into account, it's not possible to run the
bncert tool to create the certificate in advance, before you made the domain change effective. You would need to copy the certificate files and configuration from your current server to the new one if you want to test the changes first, but you would need to tweak the DNS resolution on your local computer for being able to keep HTTPS working (editing
/etc/hosts on Linux or
C:\windows\System32\drivers\etc\hosts on Windows).
My recommendation would be to use a different domain for doing the testing, then migrate to the definitive one when you are ready to do the switch.