Trying to Update User AuthenticationType via WebServices

Question

I have a .net web service that I have successfully used to update user records for other fields but when I try to use it to update the field AuthenticationType nothing happens. No error is generated. If I submit just the ExternalSystemId and the AuthenticationType (value = "User ID/PWD or Single Sign-On") it will return the user record with the AuthenticationType = null.
I also checked the Web Services Utilization List and it shows the UserUpdate actions but there are no errors.
Here is the code for the web service:
    public bool UpdateUsersInBatch(User.User1[] inUsers)
        User.UserWS_UserUpdate_Input input = null;
        User.UserWS_UserUpdate_Output output = null;
        bool retVal = false;
        string sessionId = LoginDefault();
        User.User _ws = new User.User();
        if (_useProxy) _ws.Proxy = new System.Net.WebProxy("proxyaddress", 8989);
        input = new User.UserWS_UserUpdate_Input();
        input.ListOfUser = inUsers;
        try
            _ws.Url = GetWSURL(sessionId);
            output = _ws.UserUpdate(input);
        catch (Exception ex)
            LogIt("UpdateUsersInBatch->ERROR->" + ex.ToString());
            throw;
        finally
            Logoff(ConfigurationManager.AppSettings["LoginURL"], sessionId);
        if (output.ListOfUser.Length > 0)
            retVal = true;
        return retVal;
    }

Answer 1

I also can verify that the web service is working correctly because I can submit a change to other fields including the AuthenticationType and all the other fields will update correctly just not the AuthenticationType field.