We can reassign screen flow using Reassign OOTB activity to worklist (or using pyReassignAssignment to WB/WL). But previous assignments and next assignments would be routed to previous assignee. In order to route to modified assignee change the DefaultAssignTo property value from old operator to new operator. You can find it in pxFlow - pyFlowParameters - DefaultAssignTo.
To identify flow whether it is standard flow or screenflow use pyCategory.
Thanks for your advice. I am following the suggestions but I am facing some issues with the DefaultAssignTo property, I am modifying DefaultAssignTo property setting the value of the new operator and I am checking in the clipboard to see if the change was done, and It was. But when the new operator opens the assignment DefaultAssignTo property returns to the old operator. I realized this checking the clipboard immediately after the new operator opens the assignment. As a result the next assignments will be assigned to the old operator.