As important as it was to contact Tumblr when they made that asinine, fucked up change, now that they've reversed it, you have a chance to say thanks for reversing it.
I know that you're probably still riding the high (negative) from the stress of going through yesterday's changes, and worrying about the fate of your community, and how badly the change would have affected creators. We banded together, and we let the staff know that we didn't like what they were doing, and you know what? They listened. That's part of why we're here on Tumblr and not other places that would have just shoved that down our throats regardless of the hundreds of thousands of complaints. Yeah, they fucked up. Big time. But they also backed off, and it's important to speak on that front as well if you want them to do it again later.
So, even though you may still be mad that they did it at all, take a moment and a breath, and click here. Select feedback if it isn't already selected. And just say "Hey, thanks for listening to your users, and for rolling back the change. I like my community here, and I like that you listened to us when we asked you not to do something we didn't like." It doesn't need to be elaborate. But if we want them to listen to us, making it worth listening to us is a step in the process.
Which also means that if you added a negative review somewhere due to this update, consider retracting or editing it to address the back track on the update. If you were a premium user and you cancelled in order to speak with your money to object to the change, consider speaking again now that you've been listened to.
Staff fucked up. But I'd rather have a staff that fucks up sometimes and then actually listens to its users and fixes or tries to fix it, than a staff that fucks up and pushes through to force the fuck up on everyone. So I'm going to thank them for listening this time, and I'm going to encourage them to listen again next time.