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Your account is persistent, gamertag is just the name people see. Everything will stay the same besides your screen name.
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I finally changed mine from my original tag from the halo 2 days to something more "adult" and it automatically changed on my friends friend lists. So no need to inform people. Plus it shows up in their stream you changed it.
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I don't know but I would imagine it does. It's up to you to notify those people you changed your name though.