I purchased 2 Airs in February and upgraded my personal air easily Wednesday. I went to upgrade my other Air (given to my daughter) and couldn't upgrade using my Apple ID. The machine kept rejecting my ID & password Used her ID and got charged a 2nd time! Emailed support through the itunes store and they granted a credit within hours!
Perhaps this experience (and solution) will help someone else!
Perhaps this experience (and solution) will help someone else!