To get the Component Presentations on a Page, There are two different types of component presentations, dynamic and embedded. Embedded templates are used when a component is added to a page and published only in the context of that page. Dynamic means that the component can be published out to the broker, in your case I would assume it's an embedded CP, then ...
Reference sample Code for Unpublish PageByUrl using CoreService:
static void Main(string args)
Editing for clarity.
If by decommission you mean "stop paying the support fees, but keep the software running",
Assuming what you intend to do is to continue using the software but stop paying the support & maintenance fee, that's exactly what will happen. Your current version will continue running, and everything will work as is, but: