Brand new software that-- and this seems to be missed in the wider discussion-- may be impossible to produce. It may not be possibly for Apple to comply. For instance, you can't install firmware on a locked phone.
My interpretation was that it would affect future iphones, not current ones. This is less to do with what was said, and more to do with what is practical.
It would be absolutely trivial to disable security features before selling to the customer. Afterward is more difficult.
You certainly can - you can try it out yourself by rebooting the phone while connected to USB with Home button pressed - it will reboot to the Recovery mode that (since iTunes 12 AFAIR) allows you to choose between update and restore. Should you choose 'update', it will keep your data and rewrite the system partition, all without asking for your PIN.
It would be absolutely trivial to disable security features before selling to the customer. Afterward is more difficult.