Downgrading out of Atmos?
How is this accomplished with the 2nd gen apple unit?
And what does it do to the sound experience?
Below screen capture will explain how.
Regarding changing the sound: If you have atmos speakers and you turn off the atmos feeds then no sound will come from those speakers but sound will still come from your other 5.1 speakers.
If you have say a 7.2.4 system and you shut off the atmos feeds you can change your AVR to upmix the 5.1 feed to dolby surround and sound will now come from all your speakers even your atmos speakers.