Together with the announcement of the Pre-Patch release date, Blizzard has also announced that the ability to transfer characters to other Battle.net accounts will be discontinued with The War Within Pre-Patch.
This feature is obscure and has existed since the start of the Paid Character Transfer system. It allows players to move characters to other Battle.net accounts as long as the destination Battle.net account is registered under the same name as the origin account.
If you don't have multiple Battle.net accounts, this change does not affect you.
If you do, make sure to do all transfers you're looking to do before Pre-Patch launches on July 23rd.