As an engineer, I'd want to be somewhat cautious in putting a different battery in an older car. Might be a firmware compatibility problem, or perhaps other issues. The only way to be real sure is to instrument the communications and test over a range of conditions. If this is your car, of course, this is fun. Not so fun when it is a customer's car.
Testing costs money. Even if no changes are needed or unexpected issues turn up.
So is there a positive return on investment? If not, why should Nissan spend the resources on this? Even if there is, this has to be second priority to getting the car with the 30kWh battery into production.
Now, a year or two from now, with larger margin and larger expected sales, the answer might be different. And once the production of the 24kWh battery is ramping down, there may be savings from not supporting the old battery and just switching to the newer technology.
(Disclosure: I once worked in automotive electronics design.)
Hopefully, as more automakers offer EVs, having them use and adhere to an industry-standardized battery module interface will become a competitive advantage. Some suitably detailed generalized model for batteries should be adopted and provided via a standardized interface from the battery pack itself, and the car's electronics should adapt to it. Any module, including those from 3rd party aftermarket vendors, should be capable of plugging on (at least from an electrical point of view) and either work or throw an "incompatible battery" error. It's OK if Brand X battery modules don't perform quite
as well as those from the OEM; maybe they're cheaper. Or lighter. Or simply obtainable.
LEAFs already adhere to such a standard for describing their batteries; this lets them plug into the same CHAdeMO DCFC stations that can also charge Souls, i-Mievs, or (via adapter) Teslas. I don't see that it would be such a big stretch to extend whatever battery model is presented to clue CHAdeMO stations in as to how to charge them, so that it can also describe to the car what's available for powering motors.