We won't leave your readers behind!
http://myblog.blogspot.com will redirect to your custom domain.
When you publish your blog to a custom domain, two things happen instantaneously.
- All dynamic links within the blog are changed, from "myblog.blogspot.com" to "mydomain.com".
- Blogger sets up a server based "301 Moved Permanently" for your blog.
All that said, you shouldn't expect for every search engine on the web to immediately acknowledge the new blog address. Just as establishing a search engine presence, updating one to a new address could take weeks.
With the initial setup out of the way, you may have a major amount of work yet to do, and a small but significant part needs to be done at your earliest convenience. Various third party elements that reference the blog by its URL may have to be updated. Some, if not updated immediately, may present a problem to your online reputation.
If your blog has a Google Sitemap, you'll have to use Google Webmaster Tools to re verify site ownership, and to setup a new sitemap. If you've added a Google Sitemap to your blog before, you sort of know what to expect. Also, the sitemap setup wizard was improved in early 2008, so you'll find the sitemap addition process somewhat simplified and more intuitive.
You can probably update (re add) one blog in 5 minutes, if you read the instructions. You do need to do this part fairly promptly (though after the transition period if you used the "Buy A Domain" wizard), as the sitemap is an important component in access by the Google spider, and secondarily affects your blog in Google searches.
With the sitemap updated, you can change all of the internal links in the blog, if you wish, and as time permits. Thanks to the "301 Moved Permanently", any internal references to the BlogSpot URL are simply replaced when any given link in the blog is followed. This dual personality effect will last for eternity.
Your friends web sites, that may have static references to yours (blogroll entries, generally), will likewise produce referrals to your blog, and to its new address. The static references can be updated at the convenience of the owners of the other web sites.