Block-device encryption without reinstallation

I believe that BitLocker does something like that, it would certainly be a nice feature to have.

But I'm not sure how that would work in practice: the LUKS header alone takes several MBs, so apart from shrinking the filesystem you would have to either move all the data or put the superblock and the first few MBs at the end of the partition, and then use the device mapper to make those appear at the beginning.