Just phone up retentions/cancellations, and ask them if they have a better package for you. Tell them what you want to keep or downgrade, and see what they come up with.
I renewed my contract a couple of months back, noticed Kahuna had more broadband and cost a bit less, and just phoned up and asked to switch to it. First person in India insisted I had to pay for a SH2 (not true), second person in Retentions insisted I have to have a phone (not true). I finally got to someone in retentions who put me on the right package.
If you're paying too much because of inertia (ie been on the same package for years), then call them and ask for something better. They usually have some sort of deal, even if it's just a better package that's now available at a cheaper price.
You won't get the "new customer discounts" but you should get the standard pricing that the new customer gets after his six months of discounts expire.