If you ever wanted to update all your posts in Blogger to make some sweeping change, like oh say converting to XHTML which means you need to convert <br> tags to <br/>. You might try the following:

1. Change your template to:

     <blog>
       <Blogger>
         <entry id="<$BlogItemNumber$>">
           <$BlogItemBody$>
         </entry>
       </Blogger>
     </blog>
     

2. Change your Settings so it publishes the last 999 days of entries. Also change your Blog filename to something like totalarchive
3. When you publish you now have a file called totalarchive with every post (hopefully) and all the entry ids
4. Now create a perl script which reads through each entry, grabs the id and makes the text changes. In this case a simple <[Bb][Rr]> to <br/> conversion.
5. Next use a module like Aaron's excellent Blogger.pm to call editPost to update each entry.
6. Undo your Settings changes and republish

Boy can I gloss over details. At any rate, when I'm not so tired I'll put together a perl script which does just this. This was inspired by Jako's post and the fact that at one point I converted by hand all Dave's entries to XML.