Tutorial
1. Sign in on
http://login.live.com/ and edit your last name to match a part of your gamertag (it must be a believable last name otherwise it'll be stupid)
For example: Lets say my gamertag is Sampson42, I'll go on there and change my last name to "Sampson" (Chris "Sampson")
2. Call Xbox
UK Number: 020 7365 9792 (Cost-free number)
USA Number: (800) 469-9269 (Toll free)
3. Press 2 for Xbox LIVE then 3 to speak to an xbox live representative.
4. Once talking to a representative, tell him your name (Chris "Sampson").
5. Tell him/her that you have some personal identifying information in your gamertag.
They may tell you that need Microsoft Points to change it, if they do say something like "I want my info to be kept private over xbox live and people shouldn't be able to see my last name"
If they refuse, say [I have read the code of conduct and I know that personal info isn't supposed to be in your motto, bio, and especially not your gamertag.]
If they are still refusing, just hang up and try again, or ask to speak to someone higher about it.
READ:This Obviously this isn't going to work if you gamertag is sn1p3r pr0 or some crap like that, so use some common sense?