Changing The Function Of OnMouseOut In A Div?

I want to change what an OnMouseOut even does. It is contained in a div:

<div id="div1" onMouseOut = "alert('this');">sample text</div>

once the page loads, I call onLoad="setNav('div1')";

In the head of the document is the following javascript function:

function setNav(thisDiv) {
document.getElementById(thisDiv).onMouseOut = "alert('that');";
};

I can determine that I am properly targeting the div, but I cannot change the functionality of what the onMouseOut event does for the targeted div.