Chalke replaced Goranson after the fifth season—so that Goranson could attend Vassar College—expecting audiences to embrace this chick who wasn't the Becky they'd come to love over five years.
For those with attachment issues, Goranson signed on for the eighth, and what was believed to be the last, season of the show, in which she and Chalke alternated playing the Conners' eldest kid. To the series credit, they didn't expect audiences to be that dumb, and joked about Becky's ever-changing face.
Ultimately, the joke was on the producers, as the show was renewed for one more year. Having only signed a contract for one season, Goranson relinquished all Becky duties to Chalke, and we were all expected to pretend none of the shenanigans of season eight ever happened.