They notified their users of an upcoming change and gave them about a month of warning. It'd be nice if the users could go ahead and make the change as soon as they read the email, but it's not like Tumblr is going to intentionally withhold that key detail indefinitely.
It's just a little flub, and nobody's blog is or was negatively affected, so I don't really see a need to drag out the pitchforks over it.