never rotate old tires to the front, but you can do it the other way around if you have to in a pinch. if the tire fails again in the rear you just skid out but if it fails up front there's a higher chance you'll go down

if it's otherwise in decent condition, seal or boot it and keep it in the rear. but if you want true piece of mind just invest in a new tire. i'm pretty thrift about bike parts but rubber is a worth the investment if you don't like roadrash