Userscripts.org

// ==UserScript==
// @name           YouTube HQ (Fast)
// @namespace      youtubehqfast
// @description    Switches to YouTube's HQ version quickly (no reloading)
// @include        http://*.youtube.com/watch?v=*
// @copyright      JoeSimmons
// ==/UserScript==

var mp = document.getElementById('movie_player');

var wpd = document.getElementById('watch-player-div');

var fv = mp.getAttribute("flashvars");

var fmt_map = fv.split("&fmt_map=")[1].split("&")[0];

var vq = fv.split("&vq=")[1].split("&")[0];

var p = document.createElement('embed');

p.setAttribute('flashvars', fv.replace(fmt_map, '18/512000/9/0/115').replace(vq, '2'));
p.type = mp.type;
p.src = mp.src;
p.setAttribute('id', 'movie_player');
p.setAttribute('name', 'movie_player');
p.style.bgcolor = mp.style.bgcolor;
p.quality = mp.quality;
p.allowfullscreen = mp.allowfullscreen;
p.allowscriptaccess = mp.allowscriptaccess;
p.width = mp.width;
p.height = mp.height;

wpd.removeChild(mp);
wpd.appendChild(p);