As in "spring cleaning" where we move everything out of a room in our house, so to clean the room completely, a blog owner may try exporting all comments and posts, deleting everything, then importing what was just exported.
I exported to an XML file, deleted all my blog posts and then imported - but this process has added additional digits to all of my URLs and this will break all links to me. I've switched back to my blogspot.com address but all the URLs have additional numbers like this:http://myblog.blogspot.com/2010/07/one_blog_post_579.html
As I've noted earlier in advice about deleting posts, you cannot delete then re publish a post using the same URL. Similarly, you cannot export comments and posts, delete everything, then re import and re publish - without seeing the URL of every comment, and every post, published under a new URL.
If you are really determined to make this change, your "spring cleaning" plan will have to be expanded slightly.
- Create a new blog.
- Copy the template, and all accessories, gadgets and tweaks from the current blog to the new blog.
- Import the comments and posts to the new blog.
- Finally, swap blog URLs between the two blogs.
You can do this - but note, if you do, the complications of renaming your blog. Carefully assess each issue, decide if any issue is relevant to your blog, and decide how to handle each one that is relevant.
Look, before you leap.