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.

3. Save.

After that, some time will pass for the front-end to update. but you should be done :) 

Thanks for your time!