Beginning Wednesday, any Xbox Live Gold member with a year's worth of Gold status since 2002 can pounce on one of these freed-up accounts. The first wave of accounts will be up for grabs on 11 AM PT/2 PM ET and staggered over a period of 24 hours to allow gamers in various countries an "equal chance" to snag one.
You can head to this particular Xbox.com page to prepare yourself for the great gamertag commencement, but if you've already changed your gamertag before once for free, it's going to cost a small fee to change your gamertag again here, and every subsequent time after that. If it's worth assuming an identity that feels right to you after potentially having to wait a few years to grab the perfect one, then see if you can jump in and take what's rightfully yours.
Microsoft won't be publishing the list of available gamertags, so you'll need to ready yourself for a whole lot of guesswork.