Hot on the heels of Yahoo acquiring Tumblr, the blogging service has made a major change to its custom domain settings, and should blog owners choose not to take action, it could cause their blog to go offline. Of course, blog owners who are just using Tumblr’s standard URLs will not have a problem from this change. Those that do use a custom URL need to adjust their settings before June 21st, or their domain will no longer work, which will make it difficult for readers to find their blogs.
Thankfully, Tumblr has made sure blog owners know that an adjustment is required. First, blog owners should receive an email informing them of the change. Second, an alert will show up on the top of the page when logged in. While this is certainly an annoyance, these steps should help minimize the number of blogs that end up losing their domain redirections when June 21 rolls around.
The process of updating your custom domain is quick and painless, so at the very least, Tumblr, and by extension Yahoo, is making it easy for its users. In fact, Tumblr even offers a detailed guide that will take you through the process and provide you with the information needed for the switch to take place.
Should users miss the deadline, their blog will still be accessible, but it will only be via the default Tumblr URL. Of course, users can go back after the fact and fix it, but by that point it is possible that many readers of their blog will have been lost, which could be a problem.
Source: Quick Online Tips