WHY did they switch actresses? I still don't get it! They're both pretty and they CAN act, but why the switch?

Lecy wanted to attend Vassar College(I think) then a year after she left, Sarah Chalke signed on to play the part of Becky and when we all though 1995 would be the lst season for the show.(It's was Roseanne's idea) Lecy was brought back and I think she was still attending college. In 1996 she went back to school full time and Sarah came back although she was given very little to do.

Lecy or Sarah, Becky was the best character in the show