I know that My Neighbor Totoro was released at least twice (possible three times because of some sort of airplane release) in the United States. I'm pretty sure that at there was at least a cast change between the two major US releases, but are there any other changes? Was anything censored or was any animation or lines changed?
According to the Wikipedia article, Tokuma Communications did a release in 1993, which was released on video by 20th Century Fox and dubbed again by Streamline as a special release for airlines. The Wikipedia article claims that Miyazaki would not allow any sort of editing, modification, or censorship, due to his disappointment with Warriors of the Wind, the heavily edited version of Nausicaä of the Valley of the Wind that was released in the US. The article offers no source, but this site supports the idea that Miyazaki hated Warriors of the Wind.
Fox's rights expired in 2004, and Disney acquired the license, creating a new dub. The Ghibli fan site from the link above makes the following claim about the Disney / Ghibli relationship:
No. This will not happen [editing of or changes to the Ghibli films]. Disney can not cut even one second from the films, according to the contract. Ghibli has officially stated that "With Disney's commitment to maintain the quality of the original titles, there will be no changes to music and sequences in foreign language versions." According to Mr. Suzuki, the producer of Ghibli, other companies such as Fox and Time-Warner contacted Tokuma, but Disney was the only company willing to agree to this condition, and that was the main reason why Tokuma chose Disney as a partner.
The FAQ on My Neighbor Totoro also claims that nothing was cut from the Disney release, but does state that the songs were translated into English and dubbed by an American singer, as with other Ghibli releases by Disney.
It's been a long time since I watched the Disney dub, and an even longer time since I watched the Carl Macek dub, but I'm pretty sure Disney did a new translation and wrote a new dub script. The Disney DVD also includes Japanese audio with English subtitles. I never watched Totoro subbed, but I have watched other Disney releases subbed and compared them with the dub translations, and the degree of fidelity is always very high.