If you were in a rush when you picked your StarCraft II callsign and, in hindsight, think it sucks, help's on the way. PC Gamer is reporting that Blizzard will allow one free rename. Others will cost.
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PC Gamer says Blizzard's accommodating the user base's recent demands for the name change option, but to a point. Noting that World of Warcraft's rename policy runs $10, it expects that paid name changes might be the norm sometime in the future.
You can rename your StarCraft II callsign here.
Blizzard Grants StarCraft II Accounts One Free Name Change