The short answer is that I have not seen any requirements in the NFPA code regarding replacing an existing (but working) fire alarm panel.
However, saying that, understanding the decision-making process might help make decisions.
The NFPA Code is a publication of the non-profit organization National Fire Protection Association, Inc., (the NFPA). On the one hand, the NFPA publishes books that by themselves don't have any government rule-making authority. On the other hand, the books published by the NFPA are about the best description of how to protect from fires. (OK, they are the absolute minimum requirements. But doing less than what is shown in the NFPA Code is doing less that what is needed to protect from fire.)
One of the books is the NFPA 72 Code. NFPA 72 is just number 72 out of hundreds of code books published by the NFPA.
These books are written in such a way that, rather than trying to come up with something better, many governments just point to the NFPA Code and say that is now law.
Technology keeps changing, and there are many codes and rules besides the NFPA Code. Every few years, all of them are modified. It isn't too many years before about the only way to fix a building to match the current codes is to tear it down and rebuild it from scratch. Eventually, this would apply to almost all buildings.
Rather than bulldozing the whole city every 10 to 20 years, if the codes and rules of the day are followed as the building was originally put up, the building does not have to be continually upgraded.
Often, the Grandfather Clause applies to fire alarm systems. Remember, though, it's the government that allows for a Grandfather Clause, not the NFPA.
Authority Having Jurisdiction
Many people think of the fire marshal as the Authority Having Jurisdiction (AHJ). Remember, the fire marshal is a representative of the government, and effectively interprets the rules and codes of the government. To understand the relationship between the NFPA's role and the government's role, the NFPA is a publishing house while the government makes the rules and codes that we have to live by.
Governments often add to rules in the published code. The fire marshal, as a representative of the government will explain that. Listen to the fire marshal.
There are other people that have a say about the fire alarm system.
The insurance company's representative may say that the insurance company wants more than the absolute minimum shown in the NFPA Code. The insurance company's representative has the authority to require more. That is another AHJ.
Even the owner can make more requirements than shown in the NFPA Code. The owner is another AHJ.
In the appendix to the description of the AHJ found in the front of the NFPA Code book, all of these AHJs, and many more, are listed.
Is It in the Code to Change the Panel?
As such, I have not seen those words anywhere in the NFPA. However, if any of the AHJs require the replacement, according to the NFPA, the AHJ has the authority to require the change.