A number of users have asked about preserving their current URL. We opted not to support this scenario with the Migration Tool because of the risk of breaking file URLs in the process -- but several of you have asked about the best way to do this and I promised a blog post documenting it.
Scenario: You host a blog at www.myblog.com, published via FTP. You host your blog (and only your blog) at this address, and you want to continue to use Blogger to publish to www.myblog.com using Blogger's Custom Domains feature. This walk-through will identify the steps needed to do that. Keep in mind that any files you previously hosted on www.myblog.com -- like uploaded images, PDFs, etc., whether they're www.myblog.com/resume.pdf or www.myblog.com/uploads/images/foo.jpg -- are currently hosted at www.myblog.com too. Note: When you update your domain so that www.myblog.com points to Blogger's servers, any references to those files will break unless you follow these instructions.
1. Create your "missing files" CNAME. This will be a secondary domain you will create that will act as a backup for requests that go to Blogger which result in a 404. (More on this issue here.) To create the CNAME, pick a name for this backup domain ("files" is a popular choice) and point it at your webhost's IP address. If you have a question about setting up this CNAME, check with your domain registrar and/or webhost. (It's possible, through a tool like CPanel, that your webhost has simplified tools for this step. Check with them.)
Important: pick an image that currently works at www.myblog.com and verify that it also works at files.myblog.com before proceeding. It may take up to a few hours for DNS to propagate; be patient.
2. At Blogger.com, click on "Settings | Publishing" and click on "Switch to: Custom Domain".
and type in your domain ("www.myblog.com").
3. Click "yes" under "Use a Missing Files Host" and type in the URL you picked in step 1.
4. Fill in the word verification, and click "save settings".
5. Now update the CNAME for 'www' at your domain registrar, so that 'www' points to 'ghs.google.com.' (The trailing period is important.)
Once you click "save settings", Blogger will immediately start serving requests it receives for www.myblog.com. There may be a delay of a few hours as your DNS changes propagate. That's it -- by following these steps you will successfully convert your FTP blog into a Blogger Custom Domain blog and keep the same URL you had before.