Without going too deep into technical details, when you change something inside a Pass, we check if it is still in use by one of your customers (still in its Passbook). If it is, we do some magic and transparently push the update your customer's device.
If you provided a change message in the template used to create the Pass, the user will be informed what changed in his / her Pass with an iOS notification. With a swipe on the screen from left to right, the updated Pass will pop up on the screen.