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.
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.