I'm replying with info from an older post so that this comes up as the newest. I have tested and you can switch a group assigment from the "all group members get same grade" to "individual grading" after students have submitted the assignment. The grades assigned to everyone from the initial group grading will be preserved.

For example - you give the group a grade of 85% but know that one student did not fully participate. When you switch to "grade individually" every member of the group still has 85% but now you can change the specific student's grade. There is even an option to post a comment that is specific to the student rather than to the whole group. You can also use the Excused / Missing / Late functionality of the gradebook for a specific student.