diff --git a/src/applications/differential/management/PhabricatorDifferentialMigrateHunkWorkflow.php b/src/applications/differential/management/PhabricatorDifferentialMigrateHunkWorkflow.php
--- a/src/applications/differential/management/PhabricatorDifferentialMigrateHunkWorkflow.php
+++ b/src/applications/differential/management/PhabricatorDifferentialMigrateHunkWorkflow.php
@@ -26,10 +26,20 @@
             'name' => 'all',
             'help' => pht('Migrate all hunks.'),
           ),
+          array(
+            'name' => 'auto',
+            'help' => pht('Select storage format automatically.'),
+          ),
+          array(
+            'name' => 'dry-run',
+            'help' => pht('Show planned writes but do not perform them.'),
+          ),
         ));
   }
 
   public function execute(PhutilArgumentParser $args) {
+    $is_dry_run = $args->getArg('dry-run');
+
     $id = $args->getArg('id');
     $is_all = $args->getArg('all');
 
@@ -45,14 +55,34 @@
           'with "--all".'));
     }
 
+    $is_auto = $args->getArg('auto');
     $storage = $args->getArg('to');
-    switch ($storage) {
-      case DifferentialHunk::DATATYPE_TEXT:
-      case DifferentialHunk::DATATYPE_FILE:
-        break;
-      default:
+    if ($is_auto && $storage) {
+      throw new PhutilArgumentUsageException(
+        pht(
+          'Options "--to" (to choose a specific storage format) and "--auto" '.
+          '(to select a storage format automatically) are mutually '.
+          'exclusive.'));
+    } else if (!$is_auto && !$storage) {
+      throw new PhutilArgumentUsageException(
+        pht(
+          'Use "--to" to choose a storage format, or "--auto" to select a '.
+          'format automatically.'));
+    }
+
+    $types = array(
+      DifferentialHunk::DATATYPE_TEXT,
+      DifferentialHunk::DATATYPE_FILE,
+    );
+    $types = array_fuse($types);
+    if (strlen($storage)) {
+      if (!isset($types[$storage])) {
         throw new PhutilArgumentUsageException(
-          pht('Specify a hunk storage engine with --to.'));
+          pht(
+            'Storage type "%s" is unknown. Supported types are: %s.',
+            $storage,
+            implode(', ', array_keys($types))));
+      }
     }
 
     if ($id) {
@@ -64,7 +94,7 @@
 
     foreach ($hunks as $hunk) {
       try {
-        $this->migrateHunk($hunk, $storage);
+        $this->migrateHunk($hunk, $storage, $is_auto, $is_dry_run);
       } catch (Exception $ex) {
         // If we're migrating a single hunk, just throw the exception. If
         // we're migrating multiple hunks, warn but continue.
@@ -96,31 +126,85 @@
     return $hunk;
   }
 
-  private function migrateHunk(DifferentialHunk $hunk, $format) {
+  private function migrateHunk(
+    DifferentialHunk $hunk,
+    $type,
+    $is_auto,
+    $is_dry_run) {
+
+    $old_type = $hunk->getDataType();
+
+    if ($is_auto) {
+      // By default, we're just going to keep hunks in the same storage
+      // engine. In the future, we could perhaps select large hunks stored in
+      // text engine and move them into file storage.
+      $new_type = $old_type;
+    } else {
+      $new_type = $type;
+    }
+
+    // Figure out if the storage format (e.g., plain text vs compressed)
+    // would change if we wrote this hunk anew today.
+    $old_format = $hunk->getDataFormat();
+    $new_format = $hunk->getAutomaticDataFormat();
+
+    $same_type = ($old_type === $new_type);
+    $same_format = ($old_format === $new_format);
+
+    // If we aren't going to change the storage engine and aren't going to
+    // change the storage format, just bail out.
+    if ($same_type && $same_format) {
+      $this->logInfo(
+        pht('SKIP'),
+        pht(
+          'Hunk %d is already stored in the preferred engine ("%s") '.
+          'with the preferred format ("%s").',
+          $hunk->getID(),
+          $new_type,
+          $new_format));
+      return;
+    }
+
+    if ($is_dry_run) {
+      $this->logOkay(
+        pht('DRY RUN'),
+        pht(
+          'Hunk %d would be rewritten (storage: "%s" -> "%s"; '.
+          'format: "%s" -> "%s").',
+          $hunk->getID(),
+          $old_type,
+          $new_type,
+          $old_format,
+          $new_format));
+      return;
+    }
+
     $old_data = $hunk->getChanges();
 
-    switch ($format) {
+    switch ($new_type) {
       case DifferentialHunk::DATATYPE_TEXT:
         $hunk->saveAsText();
-        $this->logOkay(
-          pht('TEXT'),
-          pht('Converted hunk to text storage.'));
         break;
       case DifferentialHunk::DATATYPE_FILE:
         $hunk->saveAsFile();
-        $this->logOkay(
-          pht('FILE'),
-          pht('Converted hunk to file storage.'));
         break;
     }
 
+    $this->logOkay(
+      pht('MIGRATE'),
+      pht(
+        'Converted hunk %d to "%s" storage (with format "%s").',
+        $hunk->getID(),
+        $new_type,
+        $hunk->getDataFormat()));
+
     $hunk = $this->loadHunk($hunk->getID());
     $new_data = $hunk->getChanges();
 
     if ($old_data !== $new_data) {
       throw new Exception(
         pht(
-          'Integrity check failed: new file data differs fom old data!'));
+          'Integrity check failed: new file data differs from old data!'));
     }
   }
 
diff --git a/src/applications/differential/storage/DifferentialHunk.php b/src/applications/differential/storage/DifferentialHunk.php
--- a/src/applications/differential/storage/DifferentialHunk.php
+++ b/src/applications/differential/storage/DifferentialHunk.php
@@ -290,14 +290,24 @@
     return array(self::DATAFORMAT_RAW, $data);
   }
 
+  public function getAutomaticDataFormat() {
+    // If the hunk is already stored deflated, just keep it deflated. This is
+    // mostly a performance improvement for "bin/differential migrate-hunk" so
+    // that we don't have to recompress all the stored hunks when looking for
+    // stray uncompressed hunks.
+    if ($this->dataFormat === self::DATAFORMAT_DEFLATED) {
+      return self::DATAFORMAT_DEFLATED;
+    }
+
+    list($format) = $this->formatDataForStorage($this->getRawData());
+
+    return $format;
+  }
+
   public function saveAsText() {
     $old_type = $this->getDataType();
     $old_data = $this->getData();
 
-    if ($old_type == self::DATATYPE_TEXT) {
-      return $this;
-    }
-
     $raw_data = $this->getRawData();
 
     $this->setDataType(self::DATATYPE_TEXT);
@@ -317,10 +327,6 @@
     $old_type = $this->getDataType();
     $old_data = $this->getData();
 
-    if ($old_type == self::DATATYPE_FILE) {
-      return $this;
-    }
-
     $raw_data = $this->getRawData();
 
     list($format, $data) = $this->formatDataForStorage($raw_data);