Say you have a cube shaped cup with the cube shaped hole.

Let's say you wanted to shrink the width of the rim on a single side. You select the inner face on that side and extrude it outward. Unfortunately the top of the rim does not move so you are left with a rim of zero height hanging over the inner edge of the cup. How would you solve this so that the top rim moves too as you push the side of the cup outward?