Found the somewhere-I-read. “Note: Changing email addresses is temporarily disabled because of ongoing changes to the DeltaChat core. It should be available again in a few months.” Some of the FAQ is a bit outdated, tho (how to contact the appstore, or how to access private keys, for instance) so “months” might be an overestimate.
I have tried using Chatmail accounts with several non-Deltachat clients, intending to write up how-tos. First you need to copy that password, and dig into the database to retrieve your keys. Receiving messages isn’t too hard, but so far I have not succeeded in sending any messages through a Chatmail server using a non-Deltachat client (there seem to be breaking differences in the format of the encrypted e-mail). This is a pity, since like SecureJoin, Chatmail compatibility would be really useful to have on other clients as well. Deltachat is standards-based, and standards become more useful as they become more widespread. WIP.
Given this, I think the assumption is that no Chatmail account needs forwarding because you’ve already notified all your contacts in your client. Your contacts in saud client are assumed to be the only people who have your public key and address.
You could, however, amalgamate your old and new accounts on one profile, I think.