I hope this message finds you well, present circumstances excluded. I am a licensed attorney with over a decade of employment law
experience. It is a pleasure to assist you today.
Severance packages are not generally governed by statute. As such, and as in this case, you do not technically have a right to force the company to give you the newer severance package now, unless they had already begun giving it to other similarly situated individuals at the time of your departure. If that was the case, then you could argue that you were treated in an inequitable fashion and perhaps discriminated against. You could legally force their hand to give you the newer severance.
That said, if this came into being after your departure and acceptance of your specific package, there is nothing that you can do legally to force them to switch you over.
That said, based on your years of service, the good faith thing to do would be for them to switch you to the newer package. As such, you should send a certified letter with a return receipt to the HR department of the company and explain the situation and formally request to be switched over if at all possible. The worst thing they could tell you is no.
Let me know if you have any other questions or comments.
Best wishes going forward!