Does Naruto Shippuuden retain the original seiyuu from Naruto? I'm just curious, does Naruto Shippuuden retain its original seiyuu from Naruto (voice actors)? Or there are certain changes with the voice actors? If there are changes, what are they and what were the reasons for changing?
The answer is: sort of.

There are a number of characters who have multiple voice actors, but those don't (as far as I can tell) change as the show goes on. 

Kakashi is a character who has a second seiyuu in Shippuden, but it is one who voices him as a child. Normally, and through both Naruto and Shippuden, he is voiced by Kazuhiko Inoue, but as a child, he is voiced by Mutsumi Tamura. 

Orochimaru has had four seiyuu: one for his normal body, one for his body as a Grass ninja, one for him in the female body, and one for him as a child. 

Deidara's voice actor in the English dub is different between his episode 135 cameo and his role in Shippuden. 

Sasori has three seiyuu: one for as an adult, one for as a child, and one for as a puppet body. 

Tobi has three seiyuu: one for when he is Tobi, one for when he is 


  Obito


(see)

and one for when he is a child. 

Shikamaru has a different seiyuu for episode 141. Normally, he's voiced by Showtaro Morikubo, but in that episode, he was voiced by Nobutoshi Canna. I can't find why that switch happened, except that Nobutoshi Canna was the stand-in. 

Konohamaru has a similar situation: he is usually voiced by Ikue Otani but Akiko Koike acted as a stand-in. 

As far as I can tell, there is nobody who specifically changes seiyuu between Naruto and Shippuden, but these are characters who have multiple seiyuu for various reasons.