Generally, your layer definitions will be set prior to beginning a project. However, there will be occasions where you need to edit a layer definition once a project has already been started. When a layer is first used in an AutoCAD drawing, that layer is created in the drawing. In order to change the layer definition on the drawing, the layer must first be purged from the drawing. This is difficult to do once a drawing is populated with pipes. Design Master Plumbing has a command to make this update easy for you.
To update existing layers on a drawing, select:
DM Plumbing 
Customization LayersUpdate Drawing Layers to Match Project List
Design Master will automatically update the layer definitions on the drawing to match the new definitions in the project list.