When encrypting the wallet, warn user that old backups will become useless.

Don't include HTML in translation strings. Do split the huge message over several lines.

Prettier lines

src/qt/askpassphrasedialog.cpp

@@ -108,7 +108,16 @@ void AskPassphraseDialog::accept()
                 if(model->setWalletEncrypted(true, newpass1))
                 {
                     QMessageBox::warning(this, tr("Wallet encrypted"),
-                                         tr("Bitcoin will close now to finish the encryption process. Remember that encrypting your wallet cannot fully protect your bitcoins from being stolen by malware infecting your computer."));
+                                         "<qt>" + 
+                                         tr("Bitcoin will close now to finish the encryption process. "
+                                         "Remember that encrypting your wallet cannot fully protect "
+                                         "your bitcoins from being stolen by malware infecting your computer.") + 
+                                         "<br><br><b>" + 
+                                         tr("IMPORTANT: Any previous backups you have made of your wallet file "
+                                         "should be replaced with the newly generated, encrypted wallet file. "
+                                         "For security reasons, previous backups of the unencrypted wallet file "
+                                         "will become useless as soon as you start using the new, encrypted wallet.") + 
+                                         "</b></qt>");
                     QApplication::quit();
                 }
                 else