[JavaScript] javascript and forms changing multiple field values

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
}
}

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