Another Yoshida post addressed a question about being unable to move to language-specific servers as they might be locked for existing players.

Quote Originally Posted by Naoki_Yoshida View Post
Does this mean that the current servers will become global servers? I’m assuming that the language servers will be considered new worlds, so players in the current server will have to wait a while to join the language servers. I believe that players will want to join the language servers with their friends as soon as they are implemented, so could you please keep that in mind.
Yes, we have already considered this scenario. We will not be treating the language servers as new worlds, so there is no need to worry.