Blizzard Entertainment has announced that a round of free realm transfers will be coming to WoW Classic. The service will not be available for all Classic realms, but only “select realms” that are highly-populated. At the moment, the character transfer is only available in the NA region. In addition, players will not be able to choose the realm they would like to move their characters to, but will be automatically assigned to a pre-selected realm if the option is taken. Whether or not realm transfers will be made available in other regions is unknown.
Players with characters on over-populated realms will see an option to choose the free transfer. Characters cannot be moved if they are guild leaders, have ongoing auctions/bids, or have mail in the mailbox.
From the official post on the WoW Classic forum:
“For approximately one day, the new realms will be restricted to only those players using a free character move. After one day, any player may create new characters there. This is to allow players using the free character move service a better chance at retaining the name of the character they’re moving.”
You can see which realms are affected and what realms player characters will be transferred to by visiting the official forums.