I am going to change the css class for each option is selected. The input block size should be adjusted though changing the css class. I checked the condition(value pass) and sure it is correct. It should be be something wrong on the overwriting the css class. I tried target.CssClass = "xxx" and Label lblEditor = (Label)e.Item.FindControl("lblEditor") to change css class, seems that it is not work. How can I overwrite the old width here?

According the option have different size input box(background):

Now:

Target(Tune in F12 for test):

var selectoption = document.getElementById("<%= swapimgValue.ClientID %>");
var optionText = selectoption.options[selectoption.selectedIndex].text;

prompt("now reading", selectoption.selectedIndex); // check option

switch (selectoption.selectedIndex) {
  case "8":
    // document.getelementbyid("<%= labSponser.ClientID %>").CssClass = "previewLabel1_1";
    // document.getelementbyid("<%= labProgram.ClientID %>").CssClass = "previewLabel1_1";
    labSponser = (Label)e.Item.FindControl("labSponser");
    labSponser.CssClass = "previewLabel1_1";
    break;

  case "9":
    .......
}

// Css classes
span.previewLabel1 {
  width: 610px;
  verflow: hidden;
  display:block;
}
                    
/* -- Switch PTK length -- */
/* 1-9W */
span.previewLabel1_1 {
  width: 330px;
  overflow: hidden;
  display:block;
}
/* 10-15W */ 
span.previewLabel1_2 {
  width: 385px;
  overflow: hidden;
  display:block;
}
/* 16-19W */     
/* 20-38W */  

// Input boxs
<tr><td valign="top" class="style3"> <asp:Label ID="labSponser" runat="server" Text="" ForeColor="#076ADB" CssClass="previewLabel1"></asp:Label></td></tr> 
<tr><td valign="top" height="8px"></td></tr> 
<tr><td valign="bottom" height="40px"> <asp:Label ID="labProgram" runat="server" Text="" ForeColor="White" CssClass="previewLabel1"></asp:Label></td></tr>