Dim data As New DataSet()
Dim tasks As New DataTable("Tasks")
Dim links As New DataTable("Links")
Sub New()
    InitializeComponent()

    tasks.Columns.Add("Id", GetType(Integer))
    tasks.Columns.Add("ParentId", GetType(Integer))
    tasks.Columns.Add("Title", GetType(String))
    tasks.Columns.Add("Start", GetType(DateTime))
    tasks.Columns.Add("End", GetType(DateTime))
    tasks.Columns.Add("Progress", GetType(Decimal))

    links.Columns.Add("StartId", GetType(Integer))
    links.Columns.Add("EndId", GetType(Integer))
    links.Columns.Add("LinkType", GetType(Integer))

    data.Tables.Add(tasks)
    data.Tables.Add(links)
    tasks.Rows.Add(1, 0, "1. Summary task title", New DateTime(2010, 10, 10), New DateTime(2010, 10, 15), 30D)
    tasks.Rows.Add(2, 1, "1.1 Task", New DateTime(2010, 10, 10), New DateTime(2010, 10, 12), 10)
    tasks.Rows.Add(3, 1, "1.2 Task", New DateTime(2010, 10, 12), New DateTime(2010, 10, 15), 20D)
    tasks.Rows.Add(4, 1, "1.3 Task", New DateTime(2010, 10, 10), New DateTime(2010, 10, 11), 20D)
    tasks.Rows.Add(5, 1, "Milestone", New DateTime(2010, 10, 15), New DateTime(2010, 10, 15), 0D)
    tasks.Rows.Add(6, 0, "2. Summary task title", New DateTime(2010, 10, 10), New DateTime(2010, 10, 13), 30D)
    tasks.Rows.Add(7, 6, "2.1 Task", New DateTime(2010, 10, 10), New DateTime(2010, 10, 11), 10)
    links.Rows.Add(2, 3, 1)
    links.Rows.Add(3, 5, 1)

    Bind()
    Me.RadGanttView1.Columns.Add("Start")
    Me.RadGanttView1.Columns.Add("End")
    Me.RadGanttView1.Columns.Add("Id")
    Me.RadGanttView1.Columns.Add("ParentId")

    Me.RadGanttView1.GanttViewElement.GraphicalViewElement.TimelineStart = New DateTime(2010, 10, 9)
End Sub

Private Sub Bind()
    Me.RadGanttView1.GanttViewElement.DataSource = Nothing
    Me.RadGanttView1.GanttViewElement.TaskDataMember = "Tasks"
    Me.RadGanttView1.GanttViewElement.ChildMember = "Id"
    Me.RadGanttView1.GanttViewElement.ParentMember = "ParentId"
    Me.RadGanttView1.GanttViewElement.TitleMember = "Title"
    Me.RadGanttView1.GanttViewElement.StartMember = "Start"
    Me.RadGanttView1.GanttViewElement.EndMember = "End"
    Me.RadGanttView1.GanttViewElement.ProgressMember = "Progress"
    Me.RadGanttView1.GanttViewElement.LinkDataMember = "Links"
    Me.RadGanttView1.GanttViewElement.LinkStartMember = "StartId"
    Me.RadGanttView1.GanttViewElement.LinkEndMember = "EndId"
    Me.RadGanttView1.GanttViewElement.LinkTypeMember = "LinkType"
    Me.RadGanttView1.GanttViewElement.DataSource = data
End Sub

Private Sub RadButtonClick(sender As Object, e As EventArgs) Handles RadButton2.Click
    tasks(3)("ParentId") = 6
    Bind()
End Sub

Workaround: rebind the ganttview after changing the parent.