To change massively the sport levels of customers go to System -> Processes -> Game Level Change
You'll see the list of all customers. Press the green arrow next to the magnifying glass to open the search options. To change the levels you can select one by one or select all of them.
Once you have the list you were looking for and all the clients you want to change selected, simply write the level you want to apply on the "New level to apply" textbox and press modify selection.