modifying forms using javascrip

[michaelh613]

I have a form which is setup traditionally using tables. I know its not Web Standard but it is an older site built by someone else in Frontpage. The form has a field category and subcategory with the ability to make a new category and subcategory. When new is chosen I want that field to turn from a select field to a text input field. I am able to do that one at a time using onchange if the variable is new call a PHP script using javascript which outputs one row. However I cannot have it change two rows at one time. It appears that is not supported using innerHTML. Does anyone know how I would do this or a tutorial I could use. I'm a newbie and have a book on this on the way.

function stateChanged()
{
if (xmlHttp.readyState==4 || xmlHttp.readyState=="complete")
{
document.getElementById("txtHint").innerHTML=xmlHttp.responseText
}