Where you have wires, always best to use them. Wired sensors are a lot cheaper than wireless and you don't have the added expense of changing batteries every 3 years or so. A lot of the existing sensors are most likely still functioning perfectly, so no need to replace unless for example you want to switch simple window contacts with sensors that will also pick up glass break or vibration (from someone trying to force them).
If you want to make changes or additions to the system, then you can add wireless sensors. The HKC systems are hybrid, and so they support both wired and wireless sensors.