Do keypool keys from an unencrypted wallet backup become unusable once I encrypt my wallet?
They don't become invalid. When you ask for a new address from the client, it will not choose any that it has already given you. This is for your privacy, because it makes it more difficult to associate those addresses with you.

In this case, we're making them as used for a different reason. To explain why, let's suppose the wallet didn't treat the previously unencrypted addresses differently. 


You generate a wallet.
You read about the importance of backups, and make a backup to an external flash drive.
You read about the importance of encrypting your wallet, and do so.
For about 6 months, you use 3 new addresses every week.
You get hacked, and you have the external flash drive plugged into your computer at the time.


Your wallet would be encrypted, but that wouldn't matter, because all of the keys that hold a balance are also sitting unencrypted on your flash drive.

In theory, you could just destroy all unencrypted copies of your wallet, but that seems more difficult than generating new keys and making a new backup, no?


  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?


Yes, you could.