if you activate the app by accident you can go to the progress page and enter any crz altitude and you will un activate.
and i was reading on the post someone did mention that the speeds will be wrong because the a/c will capture the weight at that moment....that is wrong....the a/c will assume it will land soon so the speed representation on the approach page will assume that you will be landing at your current weight now....however its not frozen.....AIRBUS is smart
so long story short you can activate approach when ever you like brother.