Turn group chat into encrypted one?

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@Minim

But fundamentally I care about the relation of keys (or whatev) to people. Is there a person-based identity that can have multiple keys?

Key-contact represents a primary key. If primary key changes, you get a new contact. If someone has two primary keys, they are represented as two key-contacts.

@Minim

Or how does key rotation work?

Key rotation usually means rotation of encryption subkey. This does not change primary key, so the contact will still be recognized as the same. There is currently no key rotation in Delta Chat, as in, it does not generate new encryption subkeys. There is more discussion of this in

Autocrypt key rotation

Delta Chat uses Autocrypt for encryption. Autocrypt standard specifies that OpenPGP key conists of a signing EdDSA identity key and encryption ECDH subkey. Delta Chat allows to import OpenPGP keys that do not conform to Autocrypt specification, but let’s assume that primary key is capable of signing. To rotate encryption key, we need to remove all existing subkeys from OpenPGP key, then generate and add a new encryption subkey. To retain the ability to decrypt messages encrypted to old…

As for replacing primary (identity) key, there is a draft draft-ietf-openpgp-replacementkey-04 - OpenPGP Key Replacement but otherwise I am not aware of anyone doing identity key replacement in OpenPGP or other messengers yet. Main reason for this OpenPGP draft is making a way to eventually migrate to OpenPGP v6 and post-quantum primary keys.