On March 21, 2006, it was discovered that Pokémon USA was going to recast the voices for the English-language dub of the Pokémon anime, from season nine onwards. Pokémon USA used the special that aired on April 29, The Mastermind of Mirage Pokémon, as a testing ground for the new voice actors. This caused a controversy in the Pokémon fan community, and many flame wars.
Early in 2006, 4Kids Entertainment let their merchandising rights laspe to dub any more Pokémon episodes. Instead, dubbing was handed to TAJ Studios, which worked on the Pokémon series before. There was a catch, however: the voice actors were be replaced were to sound like the old voices. The special episode, which aired on April 29 was used as a testing ground for the new voices. Because of this, and the discontinuation of anime in their block as they were planning to move to The CW Network, Kids WB decided not to renew the series, and Cartoon Network, the sibling cable network owned by Kids WB's owners, Time Warner, started to air the series as of September 8, 2006.
Information from Bulbapedia