I'd get a new cassette. Not all cassettes have the same number of ganged sprockets. You might also end up with a big jump otherwise.

on the chain, I'd install the cassette and then check it in Big-Big combination to see if the chain is too short before buying a new chain.

(depending on how worn your chain is you might need a new one anyway when getting a new cassette, but I haven't had this problem often, and I change cassettes from time to time to meet race requirements)