What character was played by two different people on roseanne series?

The character "Becky" was played by two different actors. Sarah Chalke was one of them, who can be seen on "scrubs". Somewhere in the 3rd or 4th season of roseanne, the original actress who played becky decided to leave the show and they replaced her with Sarah chalke. And then like a season later, the original girl that played becky decided to return to the show. No clue, why Sarah chalke gave up her role.