Does Naruto Shippuuden retain the original seiyuu from Naruto?
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.