How would Amazon generally know if a product has been switched out with an inferior product on the same listing?
Something simple like either you delete the reviews or you pay to have your changes reviewed. If it is important to you to change everything it must be worth something.
Just because something is difficult to automate doesn't mean it's not worth doing.
Sometimes it does mean that.
The number of updates to product listings is large and the proportion of updates that are scams is low. That means you need to be thorough enough to not have false positives or your false positives will outweigh actual scams by a big margin. Which means high cost per inspection. But then you have to apply the high cost inspection to all the updates that aren't scams.
It's completely plausible that the cost of doing this could exceed the cost of the scams, i.e. doing it isn't worth it.
To prevent this you need actual criminal penalties, because going to jail is a deterrent in a way that having your account closed when you can just turn around and sign up for another one is not.
The problem is, there is no guarantee that it's worth more than it costs.
Regardless, simply version the product on change and only show reviews for the current version would do the trick.