I recently came across the southern and northern redbelly daces as being in an older genus 'Chrosomus'...does anyone know when the change was made from 'Chrosomus' to 'Phoxinus' for these species?

From what I could find about this (mostly from Fishes of Tennessee), Phoxinus contains both American and European species. Originally Chrosomus was used for the American species while Phoxinus for the European species. During the 1970s there was debate about whether Chrosomus was valid or was a synonym of Phoxinus with some ichthyologists prefering one over the other. By the 1980s all Chrosomus were placed in Phoxinus and this was supported by several studies. According to Eschmeyers's Catalog of Fishes the first time American species were listed in Phoxinus is in The Atlas of North American Freshwater Fishes in 1980.