Just wanted to pop in and confirm what Kitsunè said. The Free Character Migrations that are currently available are a form of population control - with the purpose of diverting those from high population realms to specific low population realms as necessary.
The current* migrations being offered can be found here:
*These options can be changed or discontinued at any time. If you find this thread in the future, and see a different realm selection available or that character migrations are not available at all, that is why.