Updating layer comps
You can save changes to a layer comp by selecting it in the Layer Comps palette and then clicking the Update Layer Comp button 
on the bottom of the palette. This rewrites an existing layer comp, bringing it up to date. For example, if your current comp has captured a certain document configuration (that is, certain layers visible, certain styles applied to some of your layers, and certain positions of the layers pixels on the canvas), and later you modify some of the parameters, making the original comp layer obsolete, you can simply update the existing layer comp to preserve the new configuration.