Apple ID change problem.
I am trying to change my Apple ID from my Gmail email address to my Yahoo email address and it tells me that my Yahoo email address is unavailable. No one else but me is using either address as an Apple ID.
Apple doesn't say this anywhere but it is pretty common the see this issue on the forums here and it appears that if you have used an email address for some other purpose on Apple such as a notification email address that Apple won't let you use it again.
