Complex sentence: The show used "Billboard" charts in its early years, touting it as "the only source" for the countdown; then, switched to those from "Radio and Records" upon its late 1990s return.
Split into simpler sentences: The show used "Billboard" charts in its early years, touting it as "the only source" for the countdown. It subsequently switched to being based on "Radio and Records" airplay data upon its late 1990s return.