Maybe you started out using the SHA1 backend, and have now configured
git-annex to use SHA256. But files you added to the annex before still
use the SHA1 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

This stages a change to the file, which you can `git commit` like any other
change.

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

## distributed migration

When you pull changes into your repository that include migration of files,
your repository then needs to be updated to follow the migration.

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

This is done automatically by commands like `git-annex pull`.

## unused old content

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.