This is a case of evolutionary biology trying to explain away the data that otherwise was not directly expected under their model.
These fossils are often celebrated as missing links and a success story for Darwinism.
Even though the differences appear superficially striking, they do not involve any novel body plans (i.e., no new proteins, new tissues, or new organs).
That evolution, over a span of 11 million years or so, was once hailed as a “poster child” for the confirmation of Darwinian theory.
Whales are held up as a model of what unguided evolution can do in a short time: in this case, just 8 million years, or perhaps even half that.