Post a redirect to your new address
If you've moved your blog to a new BlogSpot address, then you will probably want to leave a note at your old address so your readers can update their bookmarks.
As soon as you change your settings to stop using a particular BlogSpot address, that address becomes available for anyone else to use. Luckily, that includes you. So what you'll want to do after changing your address is to create a new blog using the original address. Then you can simply post a link to your new site, along with a note to let people know what's going on. You can leave that up as long as you want, and just delete that extra blog whenever you decide you don't need it anymore. Even though it uses the same address that your main blog used to, it is a completely separate blog, so deleting it will not affect your main blog at its new address.
Note: You can also use this trick of creating a placeholder blog if you think you might want to eventually switch back to the original address. Saving the address this way will prevent anyone else from using it.