gistfile1.js

// http://stackoverflow.com/questions/3503660/upgrading-with-parameter-on-a-link-to-remote-in-rails-3
// http://railsforum.com/viewtopic.php?id=42215
// http://stackoverflow.com/questions/2508213/rails3-link-to-with-attribute

    $$('a[data-remote=true]').each(function(a){
        a.onclick = function(){
            href = a.getAttribute('href');
            url = href.split('?')[0];
            params = href.split('?')[1];
            newParam = encodeURI(eval(a.getAttribute('data-with')));

            if(params){
                params = params.split('&');
                params.each(function(p){
                    if(p.match(new RegExp('^'+newParam.split('=')[0]+'='))){
                        oldParam = p;
                    };
                });
                params = params.without(oldParam);
            }else{
                params = [];
            };

            params.push(newParam);

            a.setAttribute('href', url + '?' + params.join('&'));
        };
    });