Alicia Goranson, the actress who played the eldest Conner child Becky, left the Roseanne show in 1992 to attend Vassar College in New York. Her absence was written into the show by having Becky and her boyfriend Mark (Glenn Quinn) elope and live out of state. Goranson appeared sporadically in season five, usually via phone conversations.
The producers wanted to feature Becky more prominently in season six so Sarah Chalke was hired to replace her. Goranson was able to return to the role by season eight but had to drop out of a couple episodes and Chalke replaced her again. Goranson opted not to return for season nine so Chalke finished the season.
This recasting move invited comparisons to the classic Bewitched series in which the original Darrin (Dick York) was replaced by Dick Sargent. York had been dealing with a debilitating back injury and that led to some addiction issues.