Simplest way to move it to another subdomain would be to create the subdomain and then rename the folder it creates, rename your current folder to the new subdomain you created, rename the new folder the old subdomain name. That will set those pointers up correctly. Next you need to change your blog's internal address to the new subdomain. You may need to make an edit in the database. You can then delete the old subdomain if you like.
When I moved mine I exported everything, edited the mysql dump and then had the help desk import it back into a new database.
Not sure what you mean by 'overlaid' domain to be the default.