This doesn't make sense to me. They state that the 24kwh and 30kwh batteries are physically the same dimensions and fit in the same shape hole in the chassis, ok so the 30kwh will "bolt in" to an early car question answered.
They state that the reason it cannot actually be used with the 24kwh Leaf is because the BMS will have to be different. Correct me if I'm wrong but isn't the BMS inside the sealed battery module with the cells and an independent system from the rest of the car?
And if it's not then shouldn't that just be a matter of also changing out the BMS? Of course Nissan does not want to be bound to spend any more money on warranty replacements than needed, so I would not expect them to state the batteries are interchangeable publicly. I also know Nissan is not stupid and they would not completely re-design the Leaf for a mid-gen battery upgrade so the changes must be modular and most likely minor. For instance when you swap engines in a car you usually also swap the ECU. Maybe you will also need to change the motor controller or something else but it's almost certainly not "impossible."
The software unlock is most likely the biggest hurdle to the whole thing and that is just a matter of time until it's either in the public domain or licensed to a 3rd party/done as a service.
None of us will know until an independent Leaf specialist gets ahold of the first wrecked 30kwh Leaf, which probably won't be long!