can't Update information at the editing row

[Cheryl]

I have create a dropdownlist in datagrid, and when I
select the item in the dropdownlist. It will retrieve
information from the database and update
the "Boundcolumn". But I cant invoke
the "OnSelectedIndexChanged" when I clicked on the
dropdownlist.

<ASPATAGRID id="MyDataGrid" runat="server"
AutoGenerateColumns="False" HeaderStyle-BackColor="#aaaadd"
Font-Size="8pt" Font-Name="Verdana" CellPadding="2"
BorderColor="Black" BackColor="White"Width="100%" Font-
Names="Verdana" GridLines="Horizontal"
OnEditCommand="EditCommand" OnCancelCommand="CancelCommand"
OnUpdateCommand="UpdateCommand" nItemCommand="ItemCommand"
OnSelectedIndexChanged="changeFields">

<ASP:EditCommandColumn EditText="Edit" CancelText="Cancel"
UpdateText="OK"></ASP:EditCommandColumn>
<asp:TemplateColumn HeaderText="Stage">
<ItemTemplate><%#DataBinder.Eval
(Container.DataItem,"sStage")%>
</ItemTemplate>
<EditItemTemplate>
<aspropDownList id="StageList" runat="server"
DataSource="<%# GetVersionStage(getFileName)%>"
DataTextField="sStage" >
</aspropDownList>
</EditItemTemplate>
</asp:TemplateColumn>
<asp:BoundColumn DataField="dtDateTime"
HeaderText="Date/Time" ReadOnly="True"></asp:BoundColumn>
<asp:BoundColumn DataField="nSize" HeaderText="Size"
ReadOnly="True"></asp:BoundColumn>
</Columns>
</ASPATAGRID>