Assigning KeyCode A Value In A Document Containing A Form

Basically we are trying to simulate the tab key when the
down arrow key is pressed. (we know there are other way
to control focus flow but we use a lot of dynamic jsp fields,
that will make the flow control a nightmare, we just want
basic tabbing from the arrow key)

1)We are able to capture a onkeydown event and reasign it with another
key value with no problem within an html document without a form.

example:

var ieKey = event.keyCode;

if (ieKey == 40) {
event.keyCode = 9;
}

however as soon as we insert a form tag in the document, this stops
functioning. The asignment seems to be fine as an alert clearly shows, but the
instruction seems to be ignored. Code: