Blizzard has previously allowed one free BattleTag change per Battle.net account, but is now opening up the option to pay $10 to change the BattleTag yet again.
For those who have not yet used the Change option under Account Details in your Account Management Dashboard, you still can. The BattleTag change never expires, so you will always be able to change your name for free if you have not previously done so.
If you have already changed your name and want a new one, you will be able to for $10, and will take effect automatically. Due to Blizzard’s internal ID system, your BattleTag does not have to be unique and can instead be anything you desire so long as it follows their naming policy. Friends will immediately begin to see you listed under your new name, and all new accounts still receive one fee change.