diff --git a/Command/Unannex.hs b/Command/Unannex.hs
index 288f9da44a..2c60a23bb9 100644
--- a/Command/Unannex.hs
+++ b/Command/Unannex.hs
@@ -29,8 +29,12 @@ seek = [withFilesInGit start]
 {- The unannex subcommand undoes an add. -}
 start :: CommandStartString
 start file = isAnnexed file $ \(key, backend) -> do
-	showStart "unannex" file
-	return $ Just $ perform file key backend
+	ishere <- inAnnex key
+	if ishere
+		then do
+			showStart "unannex" file
+			return $ Just $ perform file key backend
+		else return Nothing
 
 perform :: FilePath -> Key -> Backend -> CommandPerform
 perform file key backend = do
diff --git a/debian/changelog b/debian/changelog
index bd986ad24c..7ca74f9945 100644
--- a/debian/changelog
+++ b/debian/changelog
@@ -1,3 +1,11 @@
+git-annex (0.17) UNRELEASED; urgency=low
+
+  * unannex: Now skips files whose content is not present, rather than
+    it being an error. This allows gradual conversion from one backend
+    to another by running unannex followed by add in each repository.
+
+ -- Joey Hess <joeyh@debian.org>  Sat, 08 Jan 2011 15:04:48 -0400
+
 git-annex (0.16) unstable; urgency=low
 
   * git-annex-shell: Avoid exposing any git repo config except for the
diff --git a/doc/walkthrough.mdwn b/doc/walkthrough.mdwn
index 2f868e3030..47f05ebcf4 100644
--- a/doc/walkthrough.mdwn
+++ b/doc/walkthrough.mdwn
@@ -277,6 +277,19 @@ add something like this to `.gitattributes`:
 
 	* annex.backend=SHA1
 
+## migrating between backends
+
+Perhaps you had been using the WORM backend, but now have configured
+git-annex to use SHA1 for new files. Your old files are still in WORM. How
+to migrate that content? A quick and dirty way is to use the unannex
+subcommand, which removes a file from git-annex's control, followed by
+a re-add of the file, to put it in the new backend.
+
+	# git annex unannex my_cool_big_file
+	unannex my_cool_big_file ok
+	# git annex add my_cool_big_file
+	add my_cool_big_file (checksum ...) ok
+
 ## unused data
 
 It's possible for data to accumulate in the annex that no files point to
diff --git a/test.hs b/test.hs
index ea1e0728ea..35085cc37e 100644
--- a/test.hs
+++ b/test.hs
@@ -16,6 +16,7 @@ import Data.List
 import System.IO.Error
 import System.Posix.Env
 import qualified Control.Exception.Extensible as E
+import Control.Exception (throw)
 
 import qualified GitRepo as Git
 import qualified Locations
@@ -87,10 +88,11 @@ test_unannex :: Test
 test_unannex = "git-annex unannex" ~: TestList [nocopy, withcopy]
 	where
 		nocopy = "no content" ~: intmpclonerepo $ do
-			r <- git_annex "unannex" ["-q", annexedfile] 
-			not r @? "unannex incorrectly succeeded with no copy"
-			unannexed annexedfile
+			annexed_notpresent annexedfile
+			git_annex "unannex" ["-q", annexedfile] @? "unannex failed with no copy"
+			annexed_notpresent annexedfile
 		withcopy = "with content" ~: intmpcopyrepo $ do
+			annexed_present annexedfile
 			git_annex "unannex" ["-q", annexedfile] @? "unannex failed"
 			unannexed annexedfile
 			git_annex "unannex" ["-q", annexedfile] @? "unannex failed on non-annexed file"
@@ -280,7 +282,7 @@ indir dir a = do
 		(E.try (a)::IO (Either E.SomeException ()))
 	case r of
 		Right () -> return ()
-		Left e -> error $ show e
+		Left e -> throw e
 
 setuprepo :: FilePath -> IO FilePath
 setuprepo dir = do