This is a simple, 1 - 2 - 3 exercise.
- Publish a stub blog, to the old URL.
- Determine the URL of the blog post feed, under the new URL.
- Set the post feed redirect, for the stub blog.
Setup A Stub Blog
Immediately after you re publish the blog to its new URL, you should be setting up a stub blog, to visually point your readers to the new URL. Besides visually pointing your readers, you can re point your subscribers - again using the stub blog.
Determine The URL Of The New Post Feed
Copy the updated feed URL from the blog header. Precision is essential when redirecting the feed, so let Blogger do the work for you here - determine the new post feed URL, properly.
Redirect The Post Feed, For The Old URL
Go to Settings - Other, for the stub blog, published to the old URL. Edit the "Post Feed Redirect URL", and paste the new feed URL, as just copied.
When you do this, all subscribers to your blog, under the old URL, should see the blog feed in its new home, with linked content. The search engines, indexing the blog using a sitemap based upon the old URL, should start indexing it using the new URL. This should give your blog, under the new URL, a good start towards regaining the search engine reputation that it had, under the old URL.