Then you have the answer to your question. If none of them can get the maintenance reminder to be reset and they are recommending a software upgrade, that is the current answer.

If you do go through with it, I would explicitly ask, what happens if any module fails to take the new code? If the vehicle was under warranty, then BMW would take care of it. No warranty, you take care of it. You may also want the dealer to contact BMW and see what they have. It would stand to reason that if the software needs to be upgraded to reset that reminder, then there is an issue with the software and you should not be subject to paying for it.