On Tumblr, you can change a blog’s URL name to something new as long as the new URL isn’t already in use somewhere else on the site. This rule also applies to the old URL. You can save the old URL as long as someone else hasn’t started to use it. You just need to set it up as a private secondary blog right after you make the change.
Change the URL
You change a blog’s title and URL on its settings screen. Click the gear icon on your Dashboard and select the blog that has the old URL. Click the pencil icon near the title, type the new title and click “Save.” To change the URL, click the pencil icon in the Tumblr URL section, enter the new URL and click “Save.”
Save the Old URL
Click the house icon to return to the Dashboard. Click the down arrow on the sidebar and select “Create a New Blog.” Type a title in the Title field and the old URL in the URL field. Select “Password Protect This Blog,” add a password, and then click “Create Private Blog.”
Tips and Warnings
To make the old URL and secondary blog public at any time, click the gear icon, select the blog name, go to the Password section and deselect “Password Protect This Blog.”
Information in this article applies to Tumblr as of January 2014.