Turning/unloading off jquery?
You cannot just push a newer version jQuery by adding/replacing the  tag. Drupal and some modules bundles their own JavaScript that rely on that specific version of jQuery, updating jQuery may break these JavaScript. Most of these script require the jQuery object (from jQuery 1.2.6) to be binded to $.

If really needed, you can load an newer version of jQuery in noConflict mode, treating the old jQuery as a an other library. So $ will still be the old jQuery object for the existing script. Andyou can use newer jQuery version using jQuery in your script. Properly written plugins and libraries should access the jQuery object using jQuery instead of $ and so should be loadable (after calling the noConflict() method). IF not, you may need to fix them.