When you do a firmware update, the battery in the drone at the time gets updated at the same time.
If you plug in a battery with older firmware, you'll get a pop up message on the DJI-Go screen indicating that a 'module' needs updating. Click yes and the battery will be updated. It only takes a few seconds.
What could happen if you don't update the batteries?
I haven't updated the AC or the app for a couple updates, and always boot up and fly without wifi and have yet to see the 'Inconsistent' message with 2 new of the newer batteries.