Copper is the base of chrome plating. Nickel is the second phase of it. then chrome. It's a three step process which is why it is referred to as triple plating...
+1...I don't think it's possible to nickel plate over chrome(and why would you want to),and Eminent is right about the triple chrome plating process.Three metals are used,and I also agree that the final chrome plate is possibly damaged or prehaps just really dirty.Nickel is much softer and thinner than a chrome final plating,so there's no benefit ,if it were even possible,to plate chrome with nickel.
The least invasive/abrasive method that I've used is aluminum foil/coke.(No,I'm not joking,and I'm not crazy)The coke reacts with the foil,and the aluminum,is no where near as hard as chrome,so there's no danger of scratching it,as with some chrome/metal polishes.The pre wash of Dawn dish washing liquid and water will also remove surface dirt.
I'm not saying that a nickel plated dyna is impossible,but.....with a good cleaning(no abrasive chemicals) it should prove to be chrome.Just my 2 cents.
Steve B