If you have a non-Apple email address for your ID, the last time I checked, it was possible to change your Apple ID to one of the Apple email addresses ending in .mac, .me, or .icloud; but it is not possible to change your Apple ID after you do this.
There was even a little blurb (in small grey text) on the account's editing page that it was not possible.
BTW, if you have any of those apple email addresses you don't have to worry about changing one for the other. They all work like aliases for each other; not only for sending and receiving email but as Apple ID login's
Edit: Just realized that there were four more pages of replies to be read through. The above may have already been covered. Going back to read them all now. *sigh*