Android APK Certificates - what fields are required and what if that info changes?

The content of these fields are actually pretty irrelevant and are not generally seen by anyone other than you. You can leave empty fields wherever keytool will allow without consequence later.

Once you sign an apk with a keystore and upload it to the Android Market, you cannot sign that same package with another keystore. This is obviously done to prevent someone else from replacing your app with another.

For my keystore I put my personal information in it, set the expiration date for 999 years in the future and keep it safe and backed up.

To strictly answer the question about which fields are optional: actually, all fields are optional. The only requirement is that you provide at least one (even if it's just the country code).

Here's Android Studio's explanation:

It reads:

At least one Certificate issuer field is required to be non-empty

Sure enough, with some testing, I confirmed that only one field is required.

As answered by Michael, once your app is published, you can never change it.