As a rule we do not change user ID's once the person has established an identity with thier name. With 400+ post you have. We've found that name changes lead to confusion and headaches for all involved.
Simply register your new sn and start using it. But keep in mind you'll have to start over and "lose" your posts. Also, if you still want people to know who you are, you could put something like "I used to be called (insert old sn here)" in your new sig. That's the only option I see for you, if you're serious about changing your sn.
Yea the number of posts don't bother me at all, however the deeper issue is that I realized that all posts can be pulled up from other mediums which is not something I want.
My changing your name will not make the archived posts go away. Google seems to be looking to using an archive system, so things that are changed can still be looked at (the wayback machine project does the same) regardless of how much time passes.