As far as I know the bitcoin core generates a keypair of 1000 keys derived from the master key as they are used then generate 1000 more keypairs but all derived from the same master key, isn'it?
The pre-generated keys are all invalidated upon the generation of a new master private. The updated wallet.dat with the new password will then generate another 1000 addresses using the renewed seed.
the importance of renewing the backup is important to be sure to have the backup with the password updated and not with the old one, but should not generate another keypairs different from the previous backup, where receiving coins from an address generated by the new backup is not updated if we used the wallet file from the previous backup.
The point of changing the password is to ensure that people would not be able to steal any coin if they get ahold of your possibly compromised or otherwise insecure backup. It doesn't make sense if you're trying to secure your wallet, yet someone else is still able to steal your future coins. You are not supposed to use the previous backup for any of your transactions. That is your responsibility.