Since you're using video.js, you'd be better off using its API to set the new source. That way it will also work if Flash fallback had to be used.
myPlayer.src([
{ type: "video/mp4", src: "uploads/video/demovid-01-02-2014.mp4" },
{ type: "video/webm", src: "uploads/video/demovid-01-02-2014.webm" }
]);
https://github.com/videojs/video.js/blob/master/docs/api/vjs.Player.md#src-source-