This process, along with caterpillars turning into butterflies and frogs regrowing legs (and a billion other mysteries in our biological reality) doesn’t make sense from a materialist worldview—the worldview that says things are separate, operate themselves, and are disconnected from the rest of Life, with one DNA packet code per separate, solid life form.
But major internal change—this kind of smoltification process—including completely new shapes & operating systems, is more the norm than the exception in Nature. And it is actually the norm for us humans (though we are taught the materialist worldview—that we are nouns instead of verbs).
It is the natural process of evolution—a radical transformation in which our whole Being alters, finding a much bigger space to occupy, as we drop old constrictions (limiting beliefs) in our system. And this evolutionary process is built into us, just like the salmon.
But it takes some healing, some time in the brackish waters—letting go of old patterns, old internal systems…eating some salt, as it were.