I agree 
Yes, I was also thinking about cryptsetup-reencrypt but still some setup utility would be required to make this user friendly. In some thread here there was already a tutorial for something similar which used VirtualBox on Windows so the user can still look up something in case of errors.
You could also do it like this:
Download the image + setup script . Download some iso which has cryptsetup-reencrypt installed (I’m not sure if debian live isos ship with it). Attach image + iso + usb dongle/hard drive to the VM and boot it from the iso. Run the script which invokes cryptsetup-reencrypt, transfers the image to the usb/hdd and resizes the partitions so they fill the entire hard drive.
Another option would be to use an unencrypted images and the setup utility just creates an encrypted filesystem.
Just trying to gather some ideas …