All credit goes to those who have figured this out before me, I'm just putting the steps in one place.
1. Disable S Voice. Menu > Settings > More > Application manager > All
2. Install Home2 Shortcut from Google Play Store (free!).
3. Open Home2 Shortcut and set the double tap option (Home > Home) to Google. Change your double tap response if you wish, set your Home Application to TouchWiz home, and tap the Close button.
3a. If you want the double tap to immediately activate the voice search function, set your Home > Home option to Voice Search.
4. Double tap the home button, choose Home2 Shortcut and select Always.
Congrats! You have now replaced S Voice with Google Now! But wait a sec, when you activate voice dialing from a bluetooth headset, you get the Google Voice Dialer. This isn't nearly as useful as Google Now, so that's the next item to fix!
1. Install Bluetooth Launch from Google Play Store (also free!).
2. Open Bluetooth Launch.
3. This app does not display well on our screens, but you should be able to find and tap on Google Search. This expands a list of options.
4. Select "com.google.android.googlequicksearchbox.VoiceSear chActivity"
5. Activate voice dialing from your headset, choose Bluetooth Launch and Always.
Congrats again! BT headset voice dialing will now activate Google Now Voice Search!