If your chain is stretched 1/8" over 12" you're right at the border of where the cassette will run with a new chain. I'd replace the chain immediately and see if the new chain runs with the used cassette. If so, breathe a sigh of relief. If not then either put the old chain back on and milk another few thousand miles out of both, or keep the new chain and replace the cassette also.
Next time, replace the chain when it's stretched only a bit more than 1/16" over 12". Doing so will keep the cassette in better shape and you should be able to run it with up to 3-4 chains before it too is toast.
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