So this is the situation right now!
Old key I’ll refer to as “gpg key” now imported as new primary in Delta Chat. So it’s both old and new.
In between, I was using a “Delta Chat generated key” (that started on Android but was exported to iOS).
In Emacs, I can read the new Delta Chat messages that I’ve sent since I imported the “gpg key”, and I can’t read the ones that were sent during the era when I was using the “Delta Chat generated key”. So far so awesome 
In Delta Chat, I still can’t read messages that were sent to the “gpg key” that Delta received during the “Delta Chat generated key” era. I wanna, uh, “reload” them somehow so Delta Chat can grok them.
Also I wonder if someone sends me a gpg-encrypted message (manually encrypted, not using autocrypt) to my “gpg key”, as people are wont to do, and I reply from Delta, will my response be sent encrypted? It’s always awesome seeing the li’l “padlock” but sometimes I don’t even wanna hit send for fear of it being sent in plaintext. ← this is ultimately a question about what “mutual” means in the context of autocrypt: both people using GPG, or both people using autocrypt specifically?
Since I can padlock-chat with people on K9’s autocrypt just fine.