If you build your app with the old keystore key you will discover it does not match the encrypted key that Google has and you end up with a message like:
Your Android App Bundle is signed with the wrong key. Ensure that your App Bundle is signed with the correct signing key and try again. Your App Bundle is expected to be signed with the certificate with fingerprint:
SHA1: 65:3C:AE:37
8:23:C4:...
but the certificate used to sign the App Bundle you uploaded has fingerprint:
SHA1: 37:1E
5:C4:60:B9:B4:...
I'm out of time working on this, so I used the command line to create the apk, signed it and uploaded it successfully. However it's probably only a matter of time before Google stops apk files from being uploaded. It's safer for them to extract it from the aab bundle and they can optimize your app at the same time.