|1. Go to your Dashboard.|
2. Click on Settings.
3. Click on Publishing
4. The first box down the page is where you change your portion of the Blogspot url. It is of course subject to availabilty, and they will let you know while you sit there. No long waiting.
5. Scroll down and Save Settings.
If your blog has been in existence for any length of time or if there have been very many links already made to it, you ought not to change it. You may also have to resubmit any forms you've filed with the search engines.
Don't worry about the Archive posts. All of them will be appropriately changed.