<html>
<body>
<video id='videoPlayer' width="320" height="240" controls="controls">
<source id='mp4Source' src="movie.mp4" type="video/mp4" />
<source id='oggSource' src="movie.ogg" type="video/ogg" />
</video>
<!-- You MUST give your sources tags individual ID's for the solution to work. -->
<script>
var player = document.getElementById('videoPlayer');
var mp4Vid = document.getElementById('mp4Source');
player.pause();
// Now simply set the 'src' attribute of the mp4Vid variable!!!!
// (...using the jQuery library in this case)
$(mp4Vid).attr('src', "/pathTo/newVideo.mp4");
player.load();
player.play();
</script>
</body>
</html>