In the 1973 version of Doraemon, the seiyuu for Doraemon is Nozawa Masako. However in the 1979 version of Doraemon, we have Ooyama Nobuyo voicing Doraemon. The newest Doraemon (the 2005) version has Mizuta Wasabi voicing Doraemon.
Most of the time (like 99%) a seiyuu retains the voicing roles, for example, Ishikawa Yui voices Mikasa Ackerman for all 3 seasons of Attack on Titan. Matsuoka Yoshitsugu retains his role as Kirito in SAO series.
The change from Ooyama Nobuyo to Mizuta Wasabi was probably due to Ooyama suffering from dementia. Masako Nozawa was still is good health (at least no news of her suffering from certain illness). Why did not Masako Nozawa retains her role as Doraemon in 1979?