I am not sure to understand. Once the device is unlocked, what is the obstacle to change the passcode? Needing the old one? Or - if you prefer - what happens

I am not sure to understand. Once the device is unlocked, what is the obstacle to change the passcode? Needing the old one? Or - if you prefer - what happens if you have a TouchID and forget the passcode? Do you need to reset the phone?