output: preserve legacy hash_name on save() for unchanged data

dvc/output.py

@@ -692,7 +692,7 @@ class Output:
         if self.metric:
             self.verify_metric()
 
-        if self.hash_name == "md5-dos2unix":
+        if self.hash_name == "md5-dos2unix" and self.changed_checksum():
             self.hash_name = "md5"
         if self.use_cache:
             _, self.meta, self.obj = build(

tests/func/test_commit.py

@@ -316,9 +316,12 @@ def test_commit_dos2unix(tmp_dir, dvc):
         }
     )
     legacy_content = (tmp_dir / "foo.dvc").read_text()
+    assert "hash: md5" not in legacy_content
 
-    dvc.commit("foo.dvc")
+    dvc.commit("foo.dvc", force=True)
     assert (tmp_dir / "foo.dvc").read_text() == legacy_content
+
+    tmp_dir.gen("foo", "modified")
     dvc.commit("foo.dvc", force=True)
     content = (tmp_dir / "foo.dvc").read_text()
     assert "hash: md5" in content