You don't automatically get a discount just for being a past guest. Carnival may offer Past Guest promotions and you have to be careful with those too. A Past Guest promotion may offer an upgrade, a discount, or both. If you are booking far enough in advance the Early Saver rate can be better than the Past Guest promotions. Keep in mind though, the Early Saver rate has stipulations such as a Non-refundable deposit. However; with the Early Saver rate if the price goes down you can request an adjustment. This is the only promotion that you can ask for a price adjustment. It is the trade off for the non-refundable deposit. In your case it looks like the Early Saver rate is better than the Past Guest rate.
If you book through a local travel agent they can take care of all the bookings linking them together for dining. They can also help you out with any other travel needs such as a third party travel insurance.