I've already done a state Change.
That's the way I do it.

1: Require this:
var WorkItemProxyFactory = com.ibm.team.workitem.web.cache.internal.WorkItemProxyFactory;

2: Load the wished Workitem:
this.workingCopyProxy = WorkItemProxyFactory.getWorkItemProxy({id: this.currentId, createIfNeeded: true, doNotCache: true, doNotGetFromCache:true});
this.workingCopyProxy.initialize({});

3: Check the state you have. Because you have to stick to the logic... new -> open -> done.
This means, you can't directly change from new to done.

E.g.
   var proxyState = this.workingCopyProxy.getValue({path: 'attributes.internalState.label'});
   var aValues= this.workingCopyProxy.getEditPropsValuesForAttribute('actionValues');
          
   if (proxyState == 'Done') {
           console.log('This is Done. Do nothing')
   } etc

---------------------
       _changeStartWorkingToDone: function() {
           var action_id = this._getWorkflowActionIdWCopy('Set Done', this.workingCopyProxy);
           if (action_id != '') {
               var params = this._getParamsForWorkflowAction(action_id);
               this.workingCopyProxy.setValue(params);
               this.workingCopyProxy.storeWorkItem({
                   operationMsg: 'Saving',
                   applyDelta: true,
                   onSuccess: function(parms) {
                   },
                   onError: function(error) {
                       console.log(error);
                   }
               });
           }
       },

       _getWorkflowActionIdWCopy: function(actionLabel, workingCopy) {
           var aValues = workingCopy.getEditPropsValuesForAttribute('actionValues');
           console.log(aValues.length);
           console.log(">>>>>>>>>>>>>>>><");
           var action_id = '';
           for (i in aValues) {
               if (aValues[i].label == actionLabel) {
                   action_id = aValues[i].id;
                   break;
               }
           }
           return action_id;
       },
-----------
       _getParamsForWorkflowAction: function(action_id) {
           var params = {};
           params.attributeId = 'workflowAction';
           params.isInitialization = false;
           params.path = ["attributes", "workflowAction", "id"];
           params.value = action_id;
           return params;
       },

Hy Rupam,

Maybe this link could help
https://jazz.net/wiki/bin/view/Main/CqOslcV2#POST

{
    "errorClass": "com.ibm.team.repository.common.CSRFException",
    "errorCode": 403,
    "errorTraceMarshall":