Set workItem attribs in server side follow-up condition

I am creating a follow-up / post condition which retrieves the child work item links from a parent work item. The code should traverse the child work items setting the workflow state to the same as the parent. After setting the childWorkItem.setState2(parentState), an Immutable Property Exception is thrown. This is a server-side code module extension. I am thinking I need to have a working copy of the work item to perform the update. However, I'm not sure how to establish a workingCopy in the server side code. Here is a code snippet:

// Access Repository Service
IRepositoryItemService repositoryItemService = getService(IRepositoryItemService.class);

// Get Parent state
Identifier<IState> parentState = rootWorkItem.getState2();

////........ some code .......

// Establish the child work item object
IWorkItem childWorkItem = (IWorkItem) repositoryItemService.fetchItem(childHandle,IRepositoryItemService.COMPLETE);

// Get Child State
Identifier<IState> childState = childWorkItem.getState2();

// Compare parent and child state here

// Set child state = parent
childWorkItem.setState2(parentState);