On second thought, the migration info should be mentioned when you install the game or newer patches since too many people have no idea about it:
https://www.hiveworkshop.com/threads/arthas-campaign-orc.310342/page-14#post-3341493
https://www.hiveworkshop.com/threads/arthas-campaign-orc.310342/page-14#post-3341493
History: https://liquipedia.net/warcraft/Main_Page