Maybe you started out using the WORM backend, and have now configured
git-annex to use SHA1. But files you added to the annex before still
use the WORM backend. There is a simple command that can migrate that
data:

	# git annex migrate my_cool_big_file
	migrate my_cool_big_file (checksum...) ok

You can only migrate files whose content is currently available. Other
files will be skipped.

After migrating a file to a new backend, the old content in the old backend
will still be present. That is necessary because multiple files
can point to the same content. The `git annex unused` subcommand can be
used to clear up that detritus later. Note that hard links are used,
to avoid wasting disk space.
