For Yamato: 2199 the original production was over 30 years ago so it wouldn’t be practical.
for GitS: Arise, The decision was made to recast because of the age of the original cast (Atsuko Tanaka (Motoko) was pushing 50 I believe) So since Arise occurs at the formation of Section 9 the Decision was made to go with a much younger cast, and Funimation followed suit with their casting of it as well.)