Apple won't accept my email for change to Apple ID
My Apple ID is my work email address, but I'll be leaving in a few weeks so I want to change it to my personal email, which is xxx@btinternet.com. Apple won't accept this. It tells me I already have an account in that name or it's otherwise not an email I can use. As far as I know I have never set this up as an Apple ID, and I certainly don't have a login for it.
What do I do?
Thanks
MacBook Pro, OS X Yosemite (10.10.1), Late 2013 15" model