How can I get my old XBOX Live gamertag back?

My previous gamertag was "PlanetOrtalon86" and I changed it to "PlanetO86". I want to change it back, but I'm getting an error message saying something like:

Maximum Length: 12 Characters

How do I change it back to the old gamertag?

Solution 1:

How gamertags work has changed a bit over the years, but currently the basic restrictions are:

Gamertags can be up to 12 characters long, including spaces. They can’t start with a number.

Previously, 16 characters were supported, but this changed to support suffixing:

We require at least 3 characters to auto-assign you a suffix and make sure that everybody has an equal chance to have a name they can be proud of. Many older games allowed 16 characters for displaying gamertags. We reserve three of those characters for the suffix and one for the # symbol, leaving 12 characters for gamertags.

Unfortunately, there's no way to get your old gamertag back if it doesn't meet the current requirements.