I forgot my bitcoin wallet passphrase, but I found an older backup of the wallet.dat file that was unencrypted, I tried just swapping wallet.dat but the coins did not show up, is there any way to re-scan the blockchain for transactions to get the coins I recently deposited, or is it impossible to do?
Any transactions that you made after encrypting your wallet will not be in your backup. This is because the entire look ahead keypool of 100 keys (by default) is cleared and regenerated. Thus your backup will not have any of the keys that were generated and used while your wallet was encrypted.
Even with the new deterministic wallets an unencrypted backup will not contain the information necessary to restore an encrypted wallet. The master private key is regenerated when encryption is added so the unencrypted wallet does not have that information.
