If your anode rod is 85% remaining, you don't need to replace it. It won't hurt, but it's not necessary. Theoretically, it provides protection until it is 0% remaining, although I wouldn't let it go that long. There are a number of pictures on the web that illustrate what a spent rod looks like, including this one:
What color were these "metal" chunks? I ask because if they are a blue or purple-like color, according to the Suburban factory, it's actually probably part of the porcelain lining of the tank. The lining breaks apart over time as water pressure, road jarring, and heat all do their magic, and the pieces will flow out when you flush the tank. Of course, this exposes the metal tank to water, and that will initiate the rusting process, and the heater's eventual demise. In such a situation, there's no point in replacing the water heater until you start getting rusty water, or it leaks, and a leak will begin as a pinhole. But since the pinhole is covered by the foam insulation, you'll just find a puddle on the floor. Then it's time to swap it out.
I used to store my water heater full simply to provide additional water here at our house in the event of an earthquake. But to maximize its longevity, I've gone back to storing it empty. I still keep the fresh water tank full though.
Dave