If You Are Not Planning To Renew Domain Registration, Give Your Readers Advance Notice

One of the saddest answers given in Blogger Help Forum: Something Is Broken starts with the naive query

My domain registration expired last month, and my blog is now offline. How do I redirect the domain back to the blog?

The answer here is simple, and does not provide a lot of promise, to the blog owner.

If possible, you may be able to publish the blog back to BlogSpot - but the blog will have to be re indexed from the beginning.

Once the domain registration expires, you cannot use the domain address for reputation or traffic source. Any search engine hit entries, to the domain URL, will be 404 - and there is nothing that you can do, if you no longer own the domain.

If your blog depends upon reputation, or search engine listings, for reader activity - and you are contemplating changing from a non BlogSpot domain address back to a BlogSpot address - you need to plan this change, before you start.

Successful publishing, back to BlogSpot, starts well before registration expiration.
A successful change, back to a BlogSpot URL, starts before the domain registration expires - while you can use the domain to provide reputation, and search engine hits, to the BlogSpot URL. I like to advise people to allow at least a month for any blog to be fully indexed by search engines, when a blog is new.

If the blog is mature, and is indexed under a non BlogSpot URL, the process of re indexing under the BlogSpot URL may be only slightly briefer. A mature blog will have one thing going for it, that a new blog won't - inherent reader reputation.

Reader reputation will help some blogs - not all - survive the transition.
Unfortunately, reader reputation will be good for one thing, when re indexing your blog under the BlogSpot URL - links on the readers blogs, to your blog. If your blog has readers who don't publish blogs, or their own reader list, publicly - or if you let the domain registration expire before starting the process of re indexing the blog - you won't gain a lot from reader reputation.

All of the links on your readers blogs, just like all of the links in the search engine result pages, will be

404 not found

This sight will not provide you with reader traffic, or with any reputation.

Start at least a month before registration expiration.
If you must move from a non BlogSpot URL back to a BlogSpot URL, the time to start is a month before the domain registration expires. This is one of the few times when I will tell you to use domain forwarding. Use the Domain Manager wizard, hopefully provided by the registrar, and setup a "301 Moved Permanently" redirect to the BlogSpot URL - if the domain host allows this.

You'll also want to add a notice on the blog, announcing the change - again, well before the domain registration expires. An HTML / Text notice, visible on all pages, would be best.

This blog will move back to a native BlogSpot URL on (date). Please update your bookmarks now!

Plan the migration, and keep your readers - and maybe your readers readers, who use the links on your readers blogs to find your blog.

If you wait until the last minute, you'll be starting over - completely.
If you wait until the domain registration expires, you'll have nothing to do but publish your blog with new content - and hope that the search engines will continue to pick up the new content under the new URL - the old BlogSpot address. And maybe your current readers will find your blog. Maybe.