function changeThumb()
{   
    var oldThumb = document.getElementById('oldThumb_id');
    var newThumb = document.getElementById('newThumb');
    var changeThumb = document.getElementById('changeThumb');
    
    if(changeThumb.value=='n')
    {
        oldThumb.style.display='none';
        newThumb.style.display='block';
        changeThumb.value='y';         
    }
    else
    {       
        oldThumb.style.display='block';
        newThumb.style.display='none';
        changeThumb.value='n';
    } 
}