Speaker cable aging?

Hello,

I was talking with a speaker company rep, describing my system, asking about their product and he said I might benefit from new speaker cables.

I have audioquest Midnight 3 Hyperlitz speaker cables. I bought them used 3 years ago. I have never know anything else. I bought them based of their size, name and original price. I bought them from a friend and I payed next to nothing for them ($100 canadian).

The speaker rep said that because they are about 10 years old the copper could of oxidized thus deteriorating the sound? He said that it would of happened so gradually that no one (including my friend who is the original owner) would of noticed. He recommend going with a silver cable, he said he likes Goertz.

What would you think about this. I doubt he would lie to me, but how much am I loosing through these cables? Is it worth replacing them? If so with what? UHF magazine sells Sheffield Labs cable which is solid core copper with a silver coating. They are very cheap (relatively speaking), do you think these would be an upgrade?

Thought and opinions are welcome.

Thanks,