diff --git a/src/Hpack.hs b/src/Hpack.hs
index 4482606f..bc9eb08c 100644
--- a/src/Hpack.hs
+++ b/src/Hpack.hs
@@ -6,6 +6,8 @@ module Hpack (
 #ifdef TEST
 , hpackWithVersion
 , parseVerbosity
+, extractVersion
+, parseVersion
 #endif
 ) where
 
@@ -16,12 +18,14 @@ import           Control.DeepSeq
 import           Control.Exception
 import           Control.Monad.Compat
 import           Data.List.Compat
+import           Data.Maybe
 import           Data.Version (Version)
 import qualified Data.Version as Version
 import           System.Environment
 import           System.Exit
 import           System.IO
 import           System.IO.Error
+import           Text.ParserCombinators.ReadP
 
 import           Paths_hpack (version)
 import           Hpack.Config
@@ -65,6 +69,20 @@ parseVerbosity xs = (verbose, ys)
     verbose = not (silentFlag `elem` xs)
     ys = filter (/= silentFlag) xs
 
+safeInit :: [a] -> [a]
+safeInit [] = []
+safeInit xs = init xs
+
+extractVersion :: [String] -> Maybe Version
+extractVersion = listToMaybe . mapMaybe (stripPrefix prefix >=> parseVersion . safeInit)
+  where
+    prefix = "-- This file has been generated from package.yaml by hpack version "
+
+parseVersion :: String -> Maybe Version
+parseVersion xs = case [v | (v, "") <- readP_to_S Version.parseVersion xs] of
+  [v] -> Just v
+  _ -> Nothing
+
 hpack :: FilePath -> Bool -> IO ()
 hpack = hpackWithVersion version
 
@@ -73,16 +91,20 @@ hpackWithVersion v dir verbose = do
   (warnings, name, new) <- run dir
   forM_ warnings $ \warning -> hPutStrLn stderr ("WARNING: " ++ warning)
 
-  old <- either (const Nothing) (Just . stripHeader) <$> tryJust (guard . isDoesNotExistError) (readFile name >>= (return $!!))
+  old <- either (const Nothing) (Just . splitHeader) <$> tryJust (guard . isDoesNotExistError) (readFile name >>= (return $!!))
+  let oldVersion = fmap fst old >>= extractVersion
 
-  if (old == Just (lines new)) then do
-    output (name ++ " is up-to-date")
+  if (oldVersion <= Just v) then do
+    if (fmap snd old == Just (lines new)) then do
+      output (name ++ " is up-to-date")
+    else do
+      (writeFile name $ header v ++ new)
+      output ("generated " ++ name)
   else do
-    (writeFile name $ header v ++ new)
-    output ("generated " ++ name)
+    output (name ++ " was generated with a newer version of hpack, please upgrade and try again.")
   where
-    stripHeader :: String -> [String]
-    stripHeader = dropWhile null . dropWhile ("--" `isPrefixOf`) . lines
+    splitHeader :: String -> ([String], [String])
+    splitHeader = fmap (dropWhile null) . span ("--" `isPrefixOf`) . lines
 
     output :: String -> IO ()
     output message
diff --git a/test/HpackSpec.hs b/test/HpackSpec.hs
index 180f2f1f..a0679172 100644
--- a/test/HpackSpec.hs
+++ b/test/HpackSpec.hs
@@ -3,11 +3,13 @@ module HpackSpec (spec) where
 import           Prelude ()
 import           Prelude.Compat
 
+import           Control.Monad.Compat
 import           Control.DeepSeq
-import           Data.Version (Version(..))
+import           Data.Version (Version(..), showVersion)
 
 import           Test.Hspec
 import           Test.Mockery.Directory
+import           Test.QuickCheck
 
 import           Hpack
 
@@ -24,6 +26,22 @@ spec = do
       it "returns False" $ do
         parseVerbosity ["--silent"] `shouldBe` (False, [])
 
+  describe "extractVersion" $ do
+    it "extracts Hpack version from a cabal file" $ do
+      let cabalFile = ["-- This file has been generated from package.yaml by hpack version 0.10.0."]
+      extractVersion cabalFile `shouldBe` Just (Version [0, 10, 0] [])
+
+    it "is total" $ do
+      let cabalFile = ["-- This file has been generated from package.yaml by hpack version "]
+      extractVersion cabalFile `shouldBe` Nothing
+
+  describe "parseVersion" $ do
+    it "is inverse to showVersion" $ do
+      let positive = getPositive <$> arbitrary
+      forAll (replicateM 3 positive) $ \xs -> do
+        let v = Version xs []
+        parseVersion (showVersion v) `shouldBe` Just v
+
   describe "hpackWithVersion" $ do
     context "when only the hpack version in the cabal file header changed" $ do
       it "does not write a new cabal file" $ do
@@ -33,3 +51,21 @@ spec = do
           old <- readFile "foo.cabal" >>= (return $!!)
           hpackWithVersion (makeVersion [0,10,0]) "." False
           readFile "foo.cabal" `shouldReturn` old
+
+    context "when exsting cabal file was generated with a newer version of hpack" $ do
+      it "does not re-generate" $ do
+        inTempDirectory $ do
+          writeFile "package.yaml" $ unlines [
+              "name: foo"
+            , "version: 0.1.0"
+            ]
+          hpackWithVersion (makeVersion [0,10,0]) "." False
+          old <- readFile "foo.cabal" >>= (return $!!)
+
+          writeFile "package.yaml" $ unlines [
+              "name: foo"
+            , "version: 0.2.0"
+            ]
+
+          hpackWithVersion (makeVersion [0,8,0]) "." False
+          readFile "foo.cabal" `shouldReturn` old