I am in the midst of attempting a solution. Like many, I have a long-standing Apple ID that is tied to many things: iTunes, Support, Mac App Store, etc.
This Apple ID is an email address (@sbcglobal.net) that I want to stop using.
When I started using iCloud in the fall, I set it up with my @sbcglobal.net email. Then, I set up an @me.com email address. That @me.com email address is my secondary email under my Apple ID. (It is also the email that I want to be my mail email going forward.)
And here is where I started experimenting:
First, I logged out of iTunes, then logged back in using the @me.com email. Everything worked. iTunes took me to the profile page, where all my info was the same (and my ID was listed as @sbcglobal.net). I was able to play all purchased music. It was, in fact, the same account, just accessed using @me.com rather than @sbcglobal.net during login.
Then, I went to iCloud online (the web browser version), and did the same thing. I logged out of the @sbcglobal.net ID, and logged back in with the @me.com email. Again, everything was still there (and again, my ID in the corner was listed as @sbcglobal.net).
What this means is that these two IDs are indeed tied together.
My next step will be to try signing out of iCloud on my iMac, iPhone, and iPad, and then signing back in with the @me.com email and seeing if all the data transfers correctly. I am understandably reluctant to do that, though, as I risk losing data.
After that, I will try to sign out of all the other services and sign back in with the @me.com email -- services like iTunes, Mac App Store, Face Time, iMessage, Game Center, etc. If all goes well, all data should still be the same. The only annoyance will be that I will see my Apple ID as @sbcglobal.net, even though I'm now using @me.com.
Hopefully this is a sloution.
One final note -- in iTunes, it looks like you can change your email address. I will try to change it to @me.com. Maybe that is a way to circumvent the inability to change an old Apple ID email to a new @me.com email.
We'll see what happens.