using Microsoft.TeamFoundation.Client;
using Microsoft.TeamFoundation.WorkItemTracking.Client;
using Microsoft.TeamFoundation.TestManagement.Client;
using System.Net;

 List<int> TestCaseIds_Old = new List<int> { 100, 102, 103 };
            List<int> TestCaseIds_New = new List<int> { 10023, 42102, 67103 };

            TfsTeamProjectCollection teamProjectCollection_Old = new TfsTeamProjectCollection(new Uri("OLD TFS Collection URL"));
            TfsTeamProjectCollection teamProjectCollection_New = new TfsTeamProjectCollection(new Uri("NEW TFS Collection URL"));

            ITestManagementService testManService_Old = teamProjectCollection_Old.GetService<ITestManagementService>();
            ITestManagementService testManService_New = teamProjectCollection_New.GetService<ITestManagementService>();

            var project_Old = testManService_Old.GetTeamProject("OLD Project");
            var project_New = testManService_New.GetTeamProject("NEW Project");

            for (int id = 0; id < TestCaseIds_New.Count; id++)
            {
                var testCase_Old = project_Old.TestCases.Find(TestCaseIds_Old[id]);
                var testCase_New = project_New.TestCases.Find(TestCaseIds_New[id]);
                for (int i = 0; i < testCase_Old.Data.Tables[0].Rows.Count; i++)
                {
                    var rowCollection = testCase_Old.Data.Tables[0].Rows[i].ItemArray;
                    testCase_New.Data.Tables[0].Rows.Add(rowCollection);
                }

                List<string> fileLocations = new List<string>();
                if (testCase_Old.Attachments.Count > 0)
                {
                    WorkItemStore workItemStore = teamProjectCollection_Old.GetService<WorkItemStore>();
                    WorkItem requiredWorkItem = workItemStore.GetWorkItem(TestCaseIds_Old[id]);
                    WebClient webClient = new WebClient();
                    webClient.Credentials = System.Net.CredentialCache.DefaultCredentials;
                    for (int i = 0; i < requiredWorkItem.Attachments.Count; i++)
                    {
                        webClient.DownloadFile(requiredWorkItem.Attachments[i].Uri, "D:\Attachments\" + requiredWorkItem.Attachments[i].Name);
                        testCase_New.Attachments.Add(testCase_New.CreateAttachment("D:\Attachments\" + requiredWorkItem.Attachments[i].Name, SourceFileAction.None));
                        fileLocations.Add("D:\Attachments\" + requiredWorkItem.Attachments[i].Name);
                    }

                }
                testCase_New.Save();
                for (int i = 0; i < fileLocations.Count; i++)
                {
                    File.Delete(fileLocations[0]);
                }

            }