Hi André,
In the last step you only change one object, but in you derscription it says that you want to change all objects on the list.
In that case you would have to loop over the objects in the list and use a change activity in the loop. Then commit the list afterwards.
Also double check that you use your list operations on the correct lists, this can be confusing sometimes, when you have many lists of the same entity.
Hope this helps!