The genus name hasn't been changed to Persicaria. The Polygonum genus still exists, but some of the Polygonum species, including P. pensylvanica, have been reclassified as species of Persicaria. Others have been left in the Polygonum genus or have been reclassified as species of Aconogonon, Truellum, Reynoutria, and a few other genera.
