This is possible, but does not seem like a very good or elegant solution, at least not for long term; however it's WAY quicker than a full reinstall.
All one needs to do is get the 64 bit kernel from a mirror and install it via rpm:
wget http://mirrors.coreix.net/centos/6/os/x86_64/Packages/kernel-2.6.32-431.el6.x86_64.rpm rpm -ivh --force --ignorearch kernel-2.6.32-431.el6.x86_64.rpmThat's it, now edit /boot/grub/menu.lst to make it default and off you go: reboot!