Change Textbox Value on Selected Index Changed DDL not working

HI EE

I need to make a texbox visible only when a selection is not null, (first selection), and then enable a texbox that gets populated with the current date, (user can change the date) I am using an update panel and setting up the autopostback property to true on the dropdown list, the problem is that the texbox is taking the selection on date when I insert to table....

can you please suggest what to do?

Thanks

<td style="width: 286px">
                <label for="recordtype">
                                 *Prepper:</label><asp:DropDownList 
                    ID="txtPreppin" runat="server" CssClass="padding-input" 
                    AutoPostBack="True" onselectedindexchanged="txtPreppin_SelectedIndexChanged">
                    <asp:ListItem Text="-Select-" Value=''>-Select-</asp:ListItem>
                    <asp:ListItem>Acevedo, Noe</asp:ListItem>
                </asp:DropDownList>
                <br />
                <label for="recordtype">
                                 *Date:</label>
<asp:TextBox ID="txtprepDate" runat="server" 
                    Enabled="False" ReadOnly="True"></asp:TextBox>
                <asp:CalendarExtender ID="txtprepDate_CalendarExtender" runat="server" 
                    Enabled="True" TargetControlID="txtprepDate" Format="MM/dd/yyyy">
                </asp:CalendarExtender>
               
                
               
        </td>

   protected void txtPreppin_SelectedIndexChanged(object sender, EventArgs e)
     {

         if (Page.IsPostBack)
         {

             txtprepDate.Text = DateTime.Today.ToString("MM/dd/yyy");
             txtScanDate.Text = DateTime.Today.ToString("MM/dd/yyy");


             if (txtPreppin.SelectedItem.Text != null)
             {
                 txtprepDate.Enabled = true;
                 txtprepDate.ReadOnly = false;
                                  
             }

             else
             {
                 txtprepDate.Enabled = false;
                 txtprepDate.ReadOnly = true;
             }
         }

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