Originally Posted by FBinNY

I'd definitely start with a new chain. A worn (stretched) chain rapidly wears sprockets to match it's wear age, so staying with your old chain is comparable to immediately sacrificing the first 2,500 miles of the cassette's life.

But, save the old chain, and when the new one is worn slightly more than the original, switch back, and from there on switch back and forth as the chains wear. Ideally you want to switch more often, at intervals of 500-1,500 miles so the chains and sprockets are never far apart in wear age.

I routinely rotate 3-4 chains on each bike, switching at 1,000 miles. This keeps everything matched, reduces sprocket wear, and since I never expect to run a new chain on the worn cassette, I'm not bound by the 1/16"/ft guideline for chain replacement, and can typically run the set out to 25,000+ miles.