Need to toggle visibility on label and dropdownlist in formview after selectedindexchanged event

I'm using Visual Web Developer 2010 Express with SQL Server 2008 Express, VB.

The sub below is executed when the user selects the appropriate line in DropDownList4 (SelectedIndexChanged), no problem.  What I would like to do (desperately need to do), is after the selected part PDF is opened, toggle visibility so FNameDD.visibile = False and FName.visible = True.  Adding the commands simply doesn't work.

    Protected Sub DropDownList4_SelectedIndexChanged(sender As Object, e As System.EventArgs)
        Dim PCB1 As String = String.Empty
        If FormView1.CurrentMode = FormViewMode.ReadOnly Then
            Dim FName As Label = TryCast(FormView1.FindControl("PCB_FootprintLabel"), Label)
            Dim FNameDD As DropDownList = TryCast(FormView1.FindControl("DropDownList4"), DropDownList)
            PCB1 = PCBpdf & FNameDD.SelectedItem.Text & ".pdf"
            Response.Clear()
            Response.ContentType = "application/pdf"
            Context.Response.AddHeader("Content-Disposition", "attachment=" + PCB1)
            Response.WriteFile(PCB1)
            Response.End()
        End If
    End Sub

asp for DropDownList4 (FNamedd):

<asp:DropDownList 
                            ID="DropDownList4" runat="server" AutoPostBack="True" Visible="False" 
                            onselectedindexchanged="DropDownList4_SelectedIndexChanged" >
                            <asp:ListItem>-- Select Footprint to View --</asp:ListItem>
                        </asp:DropDownList>

asp for PCB_FootprintLabel (FName):

<asp:Label ID="PCB_FootprintLabel" runat="server" 
                        Text='<%# Bind("PCB_Footprint") %>' Width="225px" />

Yesterday I had trouble with buttonclick, today labels and dropdownlists, what tomorrow?

Thanks!