No, just re-install. One partition with /boot and another with an encrypted volume-group, holding /, swap and the rest.
But before embarking on that trip, do you really need full disk encryption? I mean, the content of /usr is on any fedora-cd ;-) And when up-and-running, everything is unlocked.
The only valid reason I can think about, is that other people have physically access to your machine and could get root-access by booting from cd/dvd, and might alter your system.
It surely works, but at a performance price. And the certainty that you have to enter the LUKS-key each time you boot.