You can reassign the product of a review ( even make it a Shop Level Review) in less than 30 seconds!
For that, please first go to your Reviews Dashboard and from there:
1. Find the review you need to move; at the far right of it there is a "3-dot" icon for "more options". Please Select MOVE.
2. Look for the product you want to move to or insert its product handle. / Select "Make this review a Shop review" if that is what you prefer instead.
After that, some time will pass for the front-end to update. but you should be done :)
Thanks for your time!