I have just purchased a new 27" iMac which comes with Mountain Lion. My wife has an older 21.5" iMac and would like to upgrade from Lion to Mountain Lion. Do I have to purchase another copy from the App Store, or can I use the licence I have on the 27" iMac? Both Macs are associated with the same Apple Account, and I know that in the past that purchasing from the App Store has allowed up to 3 downloads I believe?