If you are talking about the continuous waste under the sink )plumbing from the sink drains to the wall), I would say yes, replace it. Make sure before you go any further, however, that the kitchen drain is low enough where it comes out of the wall to accomodate the deeper sink. I would recommend replacing with PVC. It is easy to work with, seals well at the slip joints and will not rust. Get a disposer kit or continuous waste and a p-trap. Buy a disposer kit if you are coming horizontally off the disposer without using the elbow. You may need to do this to keep the plumbing high enough. Buy a continuous waste if you are using the disposer elbow. If you didn't get new basket strainers, I would recommend that also.