Have you ever heard of the flu? The flu shot changes every year based on phylogenetic differentiation. The strains change every year. Therefore the shot changes.

The flu is a virus that mutates. COVID is a virus that mutates.

In fact, I'm willing to bet that you have heard of SARS and MERS, two distinct coronaviruses. So you have the basic background with which to understand that viruses mutate, requiring adjustment.