If you could ASSURE a user that the actual keys were changed and cannot be recovered then it would be safe. Still, there is always some risk that any algo used to encrypt could be broken at some point in the future. If that was the case the header/keys would be expendable in a way. I use a similar method now. Before taking LUKS externals off premises I wipe the header and place a "fake" LUKS header on the volume. I have created scripts that can wipe and replace LUKS headers perfectly in a second or two tops. Seems like this is the same as the topic of this thread.
My take is: Encrypt from outset. Don't use on-drive encryption. If you can't encrypt up-front, use physical destruction (including making it very expensive to reconstruct) The reason for this is that my experience of hard disk failure these days is that it is often catastrophic with no warning, so it's pointless looking at after-the-fact encryption (against a determined adversary). I'd also caution people to apply this to their phones and other smart devices.
