Chrome Rivals is the only server offering Transfer option freely and in an automatic way. You can move from ANI to BCU and from BCU to ANI.
To start the transfer, first you need to have your Discord account linked with an in-game account. To do that click the top-right Discord icon in-game with Discord open(needs to be Discord app, not a web page) or visit https://chromerivals.net/account/manage/connections or use
/cr account link command in Discord
When your discord is connected to an in-game account, head out to Chrome Rivals Transfer website (Account->Nation Transfer): https://chromerivals.net/account/manage/transfer or use
/cr account nation-transfer command in Discord #bot channel.
Be aware, that depending on the balance situation on the server, your transfer might be accepted or denied.