To change a Fee Earner on a case follow the steps below:
Go into the case.
Find the Case Control Screen in the Viewing Pane.
Click Update.
At the top of the screen there will a drop down labelled Fee Earner.
Select the name of New Fee Earner and click on the Save button.
Final/y, test by leaving the case and then opening it again, now on the case control screen it should be in the new Fee Earner's name.