It should be like one line in the IRC server config to change the hostname, then update a few lines to link to other servers in the network, probably new certificates for that, update a few domain name references in the MOTD, maybe edit firewall rules, probably also the round-robin/load balancer config, wait for DNS propagation. The longer-lasting pain will be for channel operators who didn't advise their users where to connect if Freenode goes down, or set up fallbacks.