If you have a rooted Samsung Galaxy S III you can easily replace the functionalists available in S Voice with “Google Now”. Just follow the below guide:
Note: Your rooted Galaxy S III should have the stock TouchWiz Android 4.0.4 ICS ROM installed and ClockworkMod Recovery should also be flashed. This guide has been implemented on the AT&T Galaxy S III variant; however it should also work on other variants as well only if you are using the stock ROM on your phone.
All the files that will be needed during the tutorial can be downloaded from this link (XDA Forums)
Step#1: To begin with, download the Google Now .zip file and save it under your the /sdcard of your phone.
Step#2: Now reboot into ClockworkMod Recovery Mode by switching off your phone and then pressing and holding down the Volume Up + Home + Power buttons simultaneously.
Step#3: At this point, head over to your SDcard where you had installed Google Now in step#1 and flash it on your phone.
Step#4: Now choose “reboot system now” from the main menu of ClockWorkMod (check that Google Now has been installed on your phone under the name “Google”).
Step#5: Now install and configure ‘Google Latitude’ on your phone.
Step#6: At this point, run and completely configure Google Now.
Step#7: Now enter into the S Voice settings and deselect Launch S Voice.
Step#8: Now install ‘home2 shortcut’ from the Google Play store.
Step#9: Once ‘home 2 shortcut’ has been installed on your phone, press the home button of your device twice and choose ‘Voice Search’ to launch ‘Google Now’. You should see a blue microphone. Now act according to the guidelines described on the screen.
Step#10: Now run Google Now once more and push the menu button and hit ‘Refresh’. Reboot your device and you are done!
Now you have successfully replaced the S Voice with Google Now on your Galaxy S III. Cheers!