Microsoft now allows you to change your Gamertag for free (once) and for a fee thereafter. The old Gamertag becomes a “ghost.” You can only find them by their new tag or by their Xbox User ID (XUID) – a permanent number attached to the account.
If you perform a lookup and can't see someone's information, they may have altered their privacy settings. xbox gamertag lookup
New gamertags are limited to 12 characters. Microsoft now allows you to change your Gamertag