These two articles are good reading material for this thread's topic:
From what I read, the 4Kids voice actors and actresses were abruptly replaced, and weren't even asked about whether or not they wanted to continue with Pokemon USA (aka, no effort was made for negotiations to take place with the old VAs), which is what really irritated me. Apparently, it originally stemmed from keeping production costs low. :/
I've become accustomed to the new voice actors, though I still prefer the old voice actors by a large margin (especially Veronica Taylor as Ash and May; and Stan Hart as Professor Oak. I tried watching BW001, In the Shadow of Zekrom!, but I just couldn't get used to Carter Cathcart as Prof. Oak ) I'm glad a lot of the old VAs regained their roles, though; I hope that trend can continue. Sarah Natochenny at least does a pretty good Ash voice nowadays, though Taylor's voice will always be classic for me (I love spotting Taylor's voice in other works like Dinosaur King ).
Thank goodness that The Mastermind of Mirage Pokémon got re-dubbed though, because the first dubbed version was horrendous, sorry to say. :<