Why the heck would you need too do that???
Are you the keeper of your country's most valuable secrets or something, or just a bit on the "Overly" careful side...
First off, you already have 256bit encryption so you would have to have something extra valuable for someone to try to break that level of protection, because it certainly would be nearly impossible to do already and I really can't see someone taking that time & effort for a basic User machine's data.
Now I'm not positive, but you very probably "could" corrupt it all by trying to encrypt data that's already been encrypted by another process, so I wouldn't even try this if I were you. If you're determined to try it tho, I would test it on some unimportant data first.
But seriously, it's almost impossible to break a 256bit encrypted drive in the 1st place. Since the encryption algorithm process is random and requires the key that's made during the encryption process to decrypt it, it's basicall y a brute force attack and that takes serious time. I think you'd notice it before they got anywhere near breaking thru.