A change from 64 to 32-bit (or vice-versa) requires a clean install... there is no way to convert from one to the other.
That being said, have you tried running in compatibility mode? Right-click on the Far Cry executable file, select Properties, then go to the tab that has compatibility options. Select Windows XP SP2 and see if you can launch your game.
Most programs designed for the 32-bit version of Windows will work on the 64-bit version of Windows. Notable exceptions are many antivirus programs.
Device drivers designed for the 32-bit version of Windows don't work on computers running a 64-bit version of Windows. If you're trying to install a printer or other device that only has 32-bit drivers available, it won't work correctly on a 64-bit version of Windows. To learn how to check for drivers, see Update a driver for hardware that isn't working properly or go to the device manufacturer's website. You can also get information about drivers by going to the Windows 7 Upgrade Advisor webpage.