Shouldn't be a big deal at all. Back up important data first then make sure you choose the correct partition for the installation. You know the rest. Fedora will install the boot loader and you'll need to manually add the existing Fedora as a menu option if you intend booting that one also. When the time comes, if you don't know how, we can help set up grub to boot the already installed 64Bit version along with the other 2.
The Bassinator © ®