Hooking in to replace the Sidebar/Widget areas

You should look into having your sidebars utilize the remove action function.

Then you can remove a sidebar and replace it in a function any number of ways.

For instance (this examples using a conditional based on a page called some-page.

function change_sidebar_yay() {
    if ( is_single('some-page') ) {
        remove_action( 'old_sidebar', 'your_function' ); //this is sidebar you want removed
        add_action('get_sidebar', 'my_get_sidebar', 11);
    }
}

http://codex.wordpress.org/Function_Reference/remove_action

The old_sidebar and your_function is the name and the function name of the sidebar you want removed, you will have to look in your theme to find the actual values the theme author used ( usually in functions.php but not always).

Also if the theme is hardcoded or does not create sidebars using hooks/actions this will not work.