If users still have the credit card used to pay for the service, I would see about issuing a charge back. Looks to be enough evidence of false advertisement, specifically the emails that free lifetime users received.
I highly doubt a bank will be happy with you trying to charge back a $3 charge made years ago. Even if that wasn't done through an intermediary like an app store that would screw you over for the chargeback.
Having dealt with chargebacks as a merchant and user, my experience is that credit card companies will either issue the money back immediately unless disputed by the merchant, and upon dispute it's up to the user to provide proof of why the chargeback is warranted. And it looks like proof is easily gotten with all the promises cited in the article.
As a user, issuing a chargeback was a last resort if merchant was giving me the run around. Haven't issued a chargeback on a purchase years old, but have done so on one from a travel/airline site about 3-6 months old.