If the existing pipework can be reused, it will be left in-situ. Ideally, the flow and return pipes from/to your boiler will be used to hook up to your heat pump, albeit with new pipework to the hot water cylinder. But there may be a requirement to change the pipework to fit to the heat pump or possibly to make the pipework bigger. But the pipework around your house should be OK – unless the installer calculates it needs to be upgraded.