In your Pipe Data, you have 6 different "Classes" of pipe to set up to your liking. If you take one of the end classes not being used in your proposed design, you can pick a class of pipe (preferably the class style it is, however it doesn't really matter if it's not being sized). You can then completely change the class name to say"Existing Mainline to Remain" and "Existing Mainline to Remove" (2 different classes there). You can then even put something in for the descriptions.
From here, when piping, just click the shortcut key to get to those classes of pipes and place away. These classes will be on their own layer and linetype, and will show up as needed in the schedule.