This seems as some potentially misleading advertising from your competitor. If the reviewer tested Winamp, then Microsoft bought the brand and rebranded Windows Media Player as Winamp 6, those prior reviews would face no value to new customers, since they are about a different product.
A review may be useful because it tells you something about the product or its context. It may tell us that the product itself is good, and even if it was rewritten from the ground up, some of the old product may have survived in the experience of those creating the new product. Or it may be appraising collaterals to the product, such as the sales team or the provided support. If all that is left is the name, unless the review point was "I like how nicely its name sounds", it may have little application. Even if it was "It is good to have a product with X feature", with the competitor also being able to do X might be interpreted as putting words in the mouth of the reviewer that they didn't say (you may consider X and X' to be basically the same, but the reviewer might consider X' completely flawed). I think that the very least you old competitor should do is to qualify those old as the old reviews with the exact version that was reviewed.
This doesn't mean however that acquiring your brand wouldn't bring benefits to your old competitor. People may be using your brand as a generic term for the kind of product you did. They may be customers of your product and want/need to upgrade, thus changing to the new version by that different company. But showing the reviews for Winamp 5.8 and showing them as applicable to Windows Media Player isn't a good move for your competitor, in my humble non-lawyer opinion.
