"One of the key points of the sales pitch involved moving a positive review to the top of the review order where a positive review would stay for the duration of the contract. This was especially effective for completing a sale if there was a negative review on top ("the first review your customers see will always be a positive one"). So if a business became a Yelp advertiser, the review order would change, but only with that one review that was moved to the very top. No reviews were deleted or otherwise manipulated."

That's probably enough to give this case legs. IANAL but have been party to an unfair business practices lawsuit, and it's very easy to step in it.