The format for the next Pokemon TCG season was announced by Play! Pokemon to be Next Destinies and beyond. This will be effective on August 28, 2013 which is a few weeks after Worlds tournament. The 2014 Season has already began which will use the new format while the Worlds competition will be using the 2013 format. Other cards that will be useable includes Black Star Promos BW33 and up. Any sets before Next Destinies will be rotated out and unable to use within official Play! Pokemon events.
The purpose of rotation is to help create a healthy playing environment while encouraging players to discover new strategies. Several old strategies will be consider unusable due to their key cards being rotated out.
Some of the popular cards to be rotated out includes: Klinklang (BLW), Crushing Hammer (EPO), Energy Switch (BLW), Gothitelle (EPO), and Eelektrik (NVI). All variations of the long time used Eel decks will be rotated out including the popular RayEels. Gothitelle/Accelgor, which won several Nationals including United States, will be unusable due to the driving force of the deck being rotated out. Darkrai decks will lose some of their key tools such as Energy Switch and Crushing Hammer as both were used to help the deck make some important plays.
Also announced was a new format for the 2015 Season. There is not much information known about this format but it sounds like it could be some sort of Unlimited/Extended Format using cards from BW on.