If you're a D2D/A2A blogger and you've been using Windows Live Writer to compose and manage your posts, migrating old posts to a new blog can be a heck of a lot easier than, say, removing your own tonsils. This is especially useful if other means of migration are unavailable.
For the uninitiated, Live Writer can be configured to work with multiple blogs on multiple platforms. This allows you to very easily cross-post, that is, publish a single post to multiple blogs. That feature also allows you to quickly and easily move content from your old blog to its new home.
If you've been using Live Writer, and you have been saving local copies of your posts, it's a simple matter of opening that local copy, selecting the new blog from the Live Writer "Weblog" menu, and then clicking "Publish."
If you haven't saved local copies, or if you are new to Live Writer, the process is only a bit more involved.
I won't go into installing and configuring Live Writer here. You have Google -- you'll figure it out.
Once you have Live Writer installed and set up to work with both your D2D/A2A blog and your new OTN blog, here's how to migrate old posts to a new home.
Now, a couple of closing points...
Exercise a little editorial judgment in selecting the posts to migrate. Since the process described here has to be repeated for each individual post, why waste time and effort on anything that doesn't offer any real substance or reader value? Those quick, short "great post by whatisname on blah-blah-blah" items aren't worth the effort.
Finally, during the transition, don't abandon your old blog. Use it to make sure your readers know that your blog will be moving, and to remind them of where they can find your new stuff. During the transition you don't want your current readers to stop paying attention. Keep the channel open and active.