If you opt to only change your ID to a new one, the new one will automatically change in your older postings. I have seen this happen quite frequently when I've changed my ID over the years. So, it's best to start completely fresh and keep your postings as generic to your situation as possible.
Above post is great advice except that you can still figure out who you were on the old posts by any quotes or when you are called out by name, which happens quite frequently.
