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So do I just request it in here? How are the chances that they will change the username for a mortal (non-elite; not well-known) member?
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Pretty good. It's not a guarantee but the Mod's are generally willing to change the User Name of anyone that has established themselves on AT.
ie. They wouldnt change your name if you only had 100posts or so as you can easily start a new name and rack up another 100 pretty quickly.
Odds are they'd grant your request.
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Also, is there any way to check which usernames are already taken? I only seem to find the options to see all members that are online and the search function to look up posts by a certain user, which does exclude many usernames (not online/no posts).
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Unfortunately, no there is not. Members wishing to change their User Name are rather infrequent so there has never been a need for it.
As a suggestion, if you think up a name you like you could do a search for posts under that name. If nothing is found it's a pretty safe bet it hasnt been taken.
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Last question: If a mod does change my username, would there be any complications with my paid subscription? >>
I can't state with an absolute certainty, but I'm 99.9% confident there would be no issues at all.