You should be able to download the image from ADC as well as the app store. Not the case?
In any case, updates are likely to be delivered via Software Update as with Leopard and Snow Leopard or with new disk images as with earlier pre-releases that can be used to clean install or upgrade your current install.
I don't see how the upgrade mechanism in the App Store could be used to upgrade the OS directly itself.
They only released the install over the app store, so no disk images this time around.. Ill just wait