receiver has 2 keys: active key and non-active key. when a key change occurs, the nonactive key is changed to a new key which WILL be active. This is done so that all receivers have time to download the new key before it becomes active - it makes the key change seemless (no inturruption on your tv screen). After a certain amount of the time, the newly-donwloaded key is made active and the receiver begins to us this new key.
so with receivers working off of an "old" key, this can only happen during a key change event and not after the change has occured.
Of course, any receiver can still display unencrypted channels (e.g. some locals) even with key that haven't rolled (not up to date)