[schoen]

The client has a checkpointing mechanism that does back up old configuration versions and can revert them. (This client feature is called the "reverter", in case you care to look at some of the code or issues related to it on our GitHub page.)

I still haven't figured out how that interacts with the automated renewal features (probably not well right now!) but the ability to revert configurations exists.

Also, please don't try the client with a live site right now, because we don't have general public availability (nobody outside of Let's Encrypt can get a cert issued from the Let's Encrypt intermediate -- you'll get one from "happy hacker fake CA" instead), and we don't have the cross-signature. We're not even quite at the beta-test stage yet, let alone the "please use our certificates on your popular public services" stage. :-)

The main exception would be if you currently don't have HTTPS enabled at all and you're in the mood to experiment to learn more about Let's Encrypt.