So I just had a fun / probably bad idea: I'd like to convert my Arch PC to use encryption on the root partition. I'll likely try copying the root filestructure to an external drive before formatting, doing a fresh install in a dm-crypt part, and then copy back. Would that work?
That's nothing. A remarkably bad idea was converting production VM images from using (a uselessly trivial) LVM setup to plain DOS partitions without touching the data, adjusting the partition table to match the LVM data extents. And even *that* worked fine for me.