Changing my Apple ID's email: address is not available

Solution 1:

Had exact same problem. Called Apple Support. Explained problem to them. Changing emails on Apple ID's is a little tricky so my call had to be bumped up to the next level of support, but the problem was quickly ironed out. And I had THREE email addresses going! LOL!

Solution 2:

From apple support: You can't use a "rescue" email address to sign in to Apple. If you try to use a rescue email to sign in, you will get the error message: "This email address is not available. Choose a different address."

I had exactly this problem, and switching to my old mac.com email address fixed the problem.