Lets Encrypt will produce a new ssl cert for your new machines.
Simply install MIAB on the new, larger machine, point the relevant name servers to it, setup www.example.com and example.com in the dns admin panel on box.example.com and you should be good to go.
You’ll need to figure out how to setup ssl on example.com and www.example.com, but that’s where the standard, https://certbot.eff.org/ instructions apply.
Certificates are associated with domain names, not individual servers or IP addresses. LetsEncrypt will happily give you another certificate for a different server with the same domain name.