I'm a little confused by this, which i found in the technical documention:
When wallet passphrase enrcyption becomes enabled, any unused keys from the keypool are flushed (marked as used) and new keys protected with encyption are added. For this reason, make a new backup of your wallet so that you will be able to recover the keys from the new key pool should access to your backups be necessary.
If I were to encrypt my wallet.dat and had a recent backup that was unencrypted do the keypool keys in the unencrypted backup become invalid?
If I hadn't made any new transactions using the new encrypted wallet would I still be able to restore the backup using the unencrypted wallet backup or would this cause problems?