gistfile1.js

// This model allows to change the id, but save/put the model to the old Id
var ChangeableIdModel = Backbone.Model.extend({
    set: function(attr, options) {
        if (attr && attr['id'] && !this.oldId ) this.oldId = this.get('id');
        Backbone.Model.prototype.set.call(this, attr, options);
        return this;
    },  
    url: function() {
        var base = getUrl(this.collection) || this.urlRoot || urlError();
        if (this.isNew()) return base;
        return base + (base.charAt(base.length - 1) == '/' ? '' : '/') + encodeURIComponent(this.oldId || this.id);
    },  
    save: function(attr, options) {
        options || (options = {});
        var model = this;
        var success = options.success;
        options.success = function(resp, status, xhr) {
            if (model.changedId) delete model.oldId;
            if (success) success(model, resp, xhr);
        };  
        Backbone.Model.prototype.save.call(this, attr, options);
    }   
});

var getUrl = function(object) {
    if (!(object && object.url)) return null;
    return _.isFunction(object.url) ? object.url() : object.url;
};

Men, can you share function urlError(). ty