How to make this jquery and add animation

[Cpt.NoobCode]

Is there a way to animate this function? so when it "expands" switches between the css it does this slowly/medium/fast. (So how do i remake this to jquery and add animation?)

    function changeCssClass(objDivID)
    {
        if(document.getElementById(objDivID).className=='normalSize')
        {
            document.getElementById(objDivID).className = 'expandedSize';
        }
        else
        {
            document.getElementById(objDivID).className = 'normalSize';
        }
    }

css code:

.normalSize { width:640px; height:360px; }

.expandedSize { width:1000px; height:480px; }

[AndrewGSW]

Try this:

http://docs.jquery.com/Tutorials:How_jQuery_Works