When App Signing Upgrade Taking effective?

Question

Now in Google PlayStore, we can upgrade our App Signing to a new key (e.g. for improved security) as per https://developer.android.com/studio/publish/app-signing#upgrade_key. This can be done yearly.

In the testing, I see the App Signing Upgrade happened immediately.

My question is, is the new signing

1. applies to all new user downloads immediately, OR
2. will it only apply to the next App Update I upload to the PlayStore?

I hope it's the second, as it provides ample time for testing etc, given one has to register the new Keys with API i.e.

If your app uses APIs, make sure that you register the certificates for your new and legacy app signing key with API providers before publishing an update, to ensure that the APIs continue working.