The term Cretaceous-Tertiary (K-T), for the boundary or transition between those two geological periods, has been in use for many generations. Then one day it was unilaterally decreed, by an unelected, unaccountable and generally unknown entity or person (a Professor, or group of them), that henceforth everyone should use the term Cretaceous-Paleogene, instead.
This is yet-another case of the Scientific Naming Follies. In this case it is notably unfortunate for science, since the general public has a high level of interest in Dinosaurs, and know that their extinction occured at the Cretaceous-Tertiary boundary.
Neither Tertiary nor Paleogene are ‘good’ names: one means ‘third’, and the other means ‘old’. That makes them – technically – “dumb” names. Dumb in the literal sense that as names, they say or tell us nothing.