As for replacing the cassette, I doubt the cassette is trashed in 400 miles. I'd put a new chain on. If it doesn't skip, no need for a new cassette. If it skips, put on a new cassette.

Other option if the bike is still shifting ok, keep running the chain until it no longer does. Both cassette and chain will wear in together. The cassette will be toast when you eventually change the chain, and you'll need to replace both chain and cassette.

Normally, you can get 3-4 chains to one cassette, so the running it in to the ground approach is not cost effective, but if you think the cassette is already significantly worn, then keeping running both is a viable option, as long as the bike is shifting well.