previousmodel.previous(attribute) During a "change" event, this method can be used to get the previous value of a changed attribute.
var bill = new Backbone.Model({
name: "Bill Smith"
});
bill.on("change:name", function(model, name) {
alert("Changed name from " + bill.previous("name") + " to " + name);
});
bill.set({name : "Bill Jones"});