Blizzard has just enabled realm transfers from Season of Mastery realms to Wrath Classic realms. If players are interested in joining Wrath Classic realms, the only avaiable transfers are to Eranikus, Yojamba, Remulos, or Atiesh for NA Realms.
Additionally, Blizzard explains that characters that do not transfer to Wrath Classic or the permanent Classic Era realms will be deleted.