This concession only applied to subscribers whose subscription renewed automatically, and this was stated in the email Findmypast sent to subscribers on or around 16th February 2016.
So it's likely that Susan has paid/will pay the new, higher price. Findmypast might, if asked nicely, refund the difference - I do know of one member who was able to get a partial refund, but you would be relying on their goodwill.