If your reason for needing it changed is pretty compelling, you can PM Laurel or Manu (the admins) and request such a change. I only suggest this because you've amassed so many posts; people who have fewer should just start over, rather than bugging a very busy admin. Many people with even thousands of posts choose to start over at some people.
The same goes for account deletion, but then the reason has to be really, really good (e.g. the member is being stalked, threatened, etc.).