With the BlogSpot address active, you can move forward to the new address, without worrying about loss of readership, or search engines indexing problems. Both the old (BlogSpot) and new (custom domain) addresses will work, for eternity. This feature, in my opinion, is one of the ideas which exemplifies the reputation of Google, for innovative design.
If you decide to move back, and publish to BlogSpot, this may not be such an easy task.
Blogger provides the BlogSpot --> custom domain redirect, to reduce readership loss from the move to the custom domain.
If you move backwards, you won't, automatically, have a redirect from the custom domain to BlogSpot. If the search engines start to index your blog using the new URL, and you publish your blog back to BlogSpot, the indexed URL will be broken.
Upon clicking on a search engine hit entry, your potential readers will see, instead of your blog,
Server Not FoundThis won't encourage new readers.
If you truly need to move back to BlogSpot, if your DNS host will permit, and if changes in Blogger don't prevent this from working, you may be able to use a "301 Redirect" from the custom domain URL to the BlogSpot URL. This will, of course, work only as long as you continue to pay for DNS hosting of the domain.
If you decided to move back to BlogSpot for financial reasons (save $10 USD / year??), you won't be able to do this.
When you move to custom domain publishing, there is some risk - and you will have some lean hit rates, at least temporarily. But the lean hit rates, when publishing the blog to a domain URL, are nothing - compared to what you'll get when you publish the blog back to BlogSpot.
My advice? Move forward, not backwards. And plan the change, before you buy the new domain, not after you do so. And give your readers warning, before moving back.