Denise's answer is great, and is exactly what I'd expect from a company that does its due diligence in operating a service at scale.

But I'd also like to clear up sounds like a misconception in your question. If you think of Dreamwidth as a house, what they're doing here is essentially just moving or replacing the front door. If that gets screwed up, it'll be hard to get in at times, but everything is still *there* inside the house. And once the new doorway is in place and the old one removed and bricked over, it should mostly "just work" from the perspective of us users (with the exception of occasional... door... maintenance?)