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.
Tobi has three seiyuu: one for when he is Tobi, one for when he is
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.
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.
Naruto has a lot of different voice actors, but mainly for other languages. In Japanese, Naruto has only had 2 voice actors:
Kogure Ema has only voiced Naruto in Naruto (before the time skip). I believe she even only did 1 technique which would be Naruto's "Sexy Technique". Takeuchi Junko is Naruto's main voice actor; she has voiced Naruto in all movies and specials, also through out Naruto and Naruto: Shippuden.
During the casting for the Japanese version of the animated adaptations, the staff sought a male voice actor for Naruto. Junko Takeuchi, a female actress, was instead chosen following a large number of auditions that involved male actors. Before recording the first episode, Takeuchi noticed several lines from the script that ended with exclamations points which helped her define Naruto's voice. She noted difficulties in transitioning from the naruto which is young to the eledest of three in the animated adaptation of Part II, as she had to record the first episode after the time skip only one week after voicing the character from before the time skip. This concerned the character's growth as Naruto started acting more mature in contrast to his attitude of a child. It was difficult for Takeuchi to voice Naruto in his Nine-Tailed Demon Fox form and during the fight against Sasuke, due to the suffering Naruto was going through. Nine years after first voicing the character, while still finding it tough to voice Naruto, Takeuchi's opinion regarding him changed to "a very reliable young man". She admires his ability to prioritize and calmly make important decisions, and believes these traits will inspire viewers worldwide. Source