Future addresses, yes.
But what about past activity? Say that a person had put $100 in BTC into an unencrypted wallet for the last 10 months, and he then encrypts the wallet (Bitcoin CORE).
Someone finds an old unencrypted wallet.
How is it possible that it would not have valid content that was retrievable by the finder?
It's not possible for any backups to be invalidated. There isn't any way to surefire make an existing backup useless without the need for a central authority to validate each backup. The old backup
will have the previously used private keys.
If someone finds an old unencrypted wallet and your old addresses still have coins in it, they can spend it. You have to send it to a new address in your new backup to deter this possibility.