To be clear: Let’s Encrypt does not offer any key recovery service. Private keys are generated on your machines and are never under any circumstances shared with Let’s Encrypt. That said, I assume you meant account recovery. Account recovery is not yet implemented. It was originally planned as a way to ensure that the Proof of Possession challenge wouldn’t completely prevent people from issuing certificates, but since PoP is not implemented, account recovery is lower priority.
However, there’s another good reason you might want to change the email address on an account: So that you get expiration emails. This is supported in the ACME protocol and in the Boulder software: POST a signed update to your account object (aka registration object) with a new value for the Contacts field. However, unfortunately this is not yet implemented in the Python client. We have an open issue for it: https://github.com/letsencrypt/letsencrypt/issues/1215.