Probá con innerHTML:
Código PHP:
<!DOCTYPE html PUBLIC "-//W3C//DTD XHTML 1.0 Transitional//EN" "http://www.w3.org/TR/xhtml1/DTD/xhtml1-transitional.dtd">
<html xmlns="http://www.w3.org/1999/xhtml">
<head>
<meta http-equiv="Content-Type" content="text/html; charset=iso-8859-1" />
<title>test</title>
</head>
<body>
<marquee scrollamount="1" direction="right">hola</marquee>
<div style="width:100px; text-align:center; padding:2px; border:1px outset #000; cursor:pointer" onclick="document.getElementsByTagName('marquee')[0].innerHTML='chau'">cambiar</div>
</body>
</html>