What makes someone a professional reviewer?
We get paid to write reviews, we are willing to hump lotsa different gear in and out of our systems and spend several hours ferreting out all the details of a component and how it compares to something else (unless you write for TAS), and we’ve had the advantage of the perspective of hearing a lot of equipment in our own rooms and systems. Plus, we can write well enough to both convey thoughts and concepts in a way that people actually like to read.
That out of the way, personally if a manufacturer would go to the trouble/expense of shipping a current and former version of a review product I would’ve been happy to write about the differences (I actually did that once). But, that’s rarely offered and would greatly increase the cost to the manufacturer with shipping costs and such. But, and much more doable and what Soundstage.com always required, was that any reviewer had to at least have a comparable piece in their review system so there was at least one applicable point of reference. Anyway, trying to do what you’re asking would fall mainly to the manufacturers and not the review publications unless a reviewer happened to have the previous iteration of a component, which happens but is relatively rare.