Help with upgrading speaker cables

As to performance diminishing with time for cables, there is no reason that the electrical characteristics of copper cable or any common synthetic insulating material used over the last twenty years would change within a human lifetime. Unless there is visible corrosion at the connections, i.e. if you are using bananna plugs or spade connectors, there is no reason for a change. And any corrosion can be corrected by retermination. The cable has the same inductance, capacitance, and resistance as the day it was made.