We changed more gradually. My oldest went independent in science for 7th grade, and for history in 8th.
With my son, I think he felt he should be separated from his younger sister because he felt he should be doing something harder than what she did since he was older--not the same thing. I just viewed it as sort of a natural desire for a young boy who was growing up, and felt it was important to let him do that. We continued to do Bible together, and we continued literature read-alouds through high-school. I sometimes missed doing history & science all together, but it also gave my younger one a bit more room to blossom. (For her, doing science independently led to all of us discovering she had a lot of interest and talent in that area--and she's hoping to go into nursing as she heads to college next year.)
Transitions can be hard, but good too.
There were a few times in high school where my kids did the same science and history again, but independently--just depended on the year and each one's needs etc...