Here's what is likely involved in changing a battery:

1. Find an FAA licensed mechanic.
2. Find a battery. Must be an FAA approved aircraft part, traceable to the original manufacturer. And there's probably only one manufacturer.
3. Get the battery and the mechanic to the airplane.
4. Probably get everybody off the plane. (If the emergency evacuation system is impaired, that's probably going to require getting everybody off.)
5. Find (and read) the aircraft maintenance procedures book.
6. Install the battery.
7. Test the battery (by running some sort of diagnostic).
8. Make a very precise entry in the aircraft's logbook, with the mechanic's signature, license number, and date, and a "return to service" authorization statement. (This part is serious stuff. The mechanic is basically putting his career in the book for the life of the aircraft. If there's ever a problem with the emergency evacuation system the FAA will come calling.)
9. Reboard everybody.

Obviously it'll take a long time to get all this stuff done. (Just the "install" part could take quite a long time.) And if you're going to deplane everyone, you might as well move them to a plane with a battery that works while the mechanics muck with the other airplane.