Android Question Google Play Console and Updating old APK with App Bundle with Wrong Keys

Hi

I apologise as this topic has been beaten to death but my problem I cannot resolve.

I have several apps written ages ago that I keep updated. Initially, I opted for Google to manage app signing etc.
All my apps were uploaded without any Private Sign Key and all as APK's - not App Bundles.

I have to update the APK apps to meet Google's new Target SDK etc. But as you all know, Google will not allow APK's to be uploaded, only App Bundles and they must be Privately Signed.

So I have generated a new Private Sign Key for the app release that needs to be updated in the Play Store, then compiled as an App Bundle.
Upon trying to upload into the Google Play Console I get the typical error stating my App Bundle was signed with a different certificate.

I can see the two public certificates which Google hold for my app on their server:

App signing key certificate
Upload key certificate

I'm totally confused. How do I get around submitting my release as a signed app bundle where the new key matches the old existing one which Google expects from the time long ago when I initially uploaded as Google managed APK's?