I actually preferred the film to the show. I think the film had different voices.
Well, as far as I know, the Barnyard movie had mostly professional voice actors. Especially of note it had Cam Clarke and Rob Paulsen in it. That's half the original TMNT right there. I know the pig was the same guy who played Bushroot and Uncle Ted. Seems Wanda Sykes reprised her role. I think the only voice actor they had to replace was Kevin James. The guy they got doesn't sound much like him, though.
It's usually like that for most films that have spin-off shows. They don't really want to pay the actors large amounts of money for each episode, so they just get new voice actors. Look at Penguins of Madagascar, or Mater's Tall Tales to name two.
Funny thing about Penguins of Madagascar. While it's obvious they had to replace Sacha Baron Cohen, Cedric the Entertainer, and Ben Stiller (when he had that appearance in Revenge of Dr. Blowhole), they actually upgraded 2 of the Penguins with professional voice actors. Originally, they were just Dreamworks animators. The biggest change was Kowalski, who had a (frankly better) voice performed by Jeff Glenn Bennett.
But Mort had the same voice actor with Andy Richter. Not to mention a few special celebrity voice guests.
Another interesting change... the Monsters VS Aliens TV series replaced Will Arnett with Deidrich Bader. A sitcom actor turned voice actor. And while I'm on a roll, Monkey in the Kung Fu Panda series replaces Jackie Chan with the guy who
played Jackie Chan in the Jackie Chan Adventures show. How'bout that?