The greatest limitation on a plants life is the reactor vessel itself, which becomes embrittled due to neutron exposure.
This exposure, over time causes that thick piece of metal to become less ductile and more hard or brittle. This is most important during temperature changes with cooldown being the most limiting.
So, a vessel is designed to have a useful lifespan based on the amount of time it is exposed to that high neutron flux. A plant is able to show, by run time and power levels, what the exposure has been for their vessel and IF it is till within that lifespan, it can request license extension.
Additionally, some palnts even run small metal samples that are analyzed to mimic the condition of the vessel, with an eye toward demonstrating the conditions that can lead to license extension.
As noted there are also other upgrades that would be required over time. Most plants upgrade in an almost continual basis.
There is nothing underhanded in this process.
Thanks for the intelligent response.
My concern about the article is that is minimizes the hardware portion of the re licensing process. I worked at Westinghouse for several years on reactor upgrade projects. In order for the WE plants to get a 20 year extension they had to replace the whole portion of the reactor from the head flange upwards. Lots of new equipment and engineering involved, hardly a rubber stamp.