> Can the PIN change?

You can change it in the app, yes.

> How to issue new key if needed?

I think you’ll have to reissue your ID.

There’s also digi-ID (similar e-signature certificate on a card, but without any ID features), Mobiil-ID (e-signature on a SIM-card, no idea how it works), Smart-ID (in app, tied to secure storage in Android/iOS, cross-signed by the server which is supposed to check the device somehow) and probably something else I don’t remember. All of these are independent options, so you can, for example, revoke your Mobiil-ID if you lose your phone, and still use the your main ID card to sign things.