You need to file a support ticket. They have a whole category "account issues and user name changes" so I'd imagine staff would be thrilled to deal with an easily solved problem for a change.
http://support.chess.com/Tickets/Submit
Changing Username
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I'm trying to change my username so that the characters are no different, except that 2 of them are uppercase. When I attempt to do so, it says that the username is already taken. Can the computer not distinguish my current username from what I want to change it to? (mycoclairvoyance to MycoClairvoyance) Or is there an existing username the same as mine but with those letters uppercase?