Unchanged in what sense?
Consider the extant species of King Crab; morphologically they're very similar but genetically they're wildly different - as different as any other two species that diverged as far back. Morphological stasis does not mean that species have remained unchanged only that whatever change has occurred cannot be deduced from the fossil record.
This should not be surprising for two reasons. Firstly, studies on microbes have found that where a gene's function is inhibited by a mutation it is most often not restored by reversing that change but by a new mutation - at a different location - that suppresses the first change. Secondly, much of what goes on in an organism is simply not fossilisable: your immune system, the connections in your brain, the details of your metabolism, the tuning of your digestive system, the list goes on and on.