diff --git a/heudiconv/bids.py b/heudiconv/bids.py
index 97dfdec9..62795b20 100644
--- a/heudiconv/bids.py
+++ b/heudiconv/bids.py
@@ -76,6 +76,9 @@ def populate_bids_templates(path, defaults={}):
     #                    'Manufacturer', 'SliceTiming', ''}
     for fpath in find_files('.*_task-.*\_bold\.json', topdir=path,
                         exclude_vcs=True, exclude="/\.(datalad|heudiconv)/"):
+        #
+        # According to BIDS spec I think both _task AND _acq (may be more? _rec, _dir, ...?) should be retained?
+        # TODO: if we are to fix it, then old ones (without _acq) should be removed first
         task = re.sub('.*_(task-[^_\.]*(_acq-[^_\.]*)?)_.*', r'\1', fpath)
         json_ = load_json(fpath)
         if task not in tasks:
@@ -95,17 +98,33 @@ def populate_bids_templates(path, defaults={}):
             lgr.debug("Generating %s", events_file)
             with open(events_file, 'w') as f:
                 f.write("onset\tduration\ttrial_type\tresponse_time\tstim_file\tTODO -- fill in rows and add more tab-separated columns if desired")
+
     # extract tasks files stubs
     for task_acq, fields in tasks.items():
         task_file = op.join(path, task_acq + '_bold.json')
-        # do not touch any existing thing, it may be precious
-        if not op.lexists(task_file):
-            lgr.debug("Generating %s", task_file)
-            fields["TaskName"] = ("TODO: full task name for %s" %
-                                  task_acq.split('_')[0].split('-')[1])
-            fields["CogAtlasID"] = "TODO"
-            with open(task_file, 'w') as f:
-                f.write(json_dumps_pretty(fields, indent=2, sort_keys=True))
+        # Since we are pulling all unique fields we have to possibly
+        # rewrite this file to guarantee consistency.
+        # See https://github.com/nipy/heudiconv/issues/277 for a usecase/bug
+        # when we didn't touch existing one.
+        # But the fields we enter (TaskName and CogAtlasID) might need need
+        # to be populated from the file if it already exists
+        placeholders = {
+            "TaskName": ("TODO: full task name for %s" %
+                           task_acq.split('_')[0].split('-')[1]),
+            "CogAtlasID": "TODO",
+        }
+        if op.lexists(task_file):
+            j = load_json(task_file)
+            # Retain possibly modified placeholder fields
+            for f in placeholders:
+                if f in j:
+                    placeholders[f] = j[f]
+            act = "Regenerating"
+        else:
+            act = "Generating"
+        lgr.debug("%s %s", act, task_file)
+        fields.update(placeholders)
+        save_json(fields, indent=2, sort_keys=True, pretty=True)
 
 
 def tuneup_bids_json_files(json_files):
diff --git a/heudiconv/utils.py b/heudiconv/utils.py
index 3539a5a9..e40806a6 100644
--- a/heudiconv/utils.py
+++ b/heudiconv/utils.py
@@ -171,7 +171,7 @@ def assure_no_file_exists(path):
         os.unlink(path)
 
 
-def save_json(filename, data, indent=4):
+def save_json(filename, data, indent=4, sort_keys=True, pretty=False):
     """Save data to a json file
 
     Parameters
@@ -180,11 +180,17 @@ def save_json(filename, data, indent=4):
         Filename to save data in.
     data : dict
         Dictionary to save in json file.
+    indent : int, optional
+    sort_keys : bool, optional
+    pretty : bool, optional
 
     """
     assure_no_file_exists(filename)
     with open(filename, 'w') as fp:
-        fp.write(_canonical_dumps(data, sort_keys=True, indent=indent))
+        fp.write(
+            (json_dumps_pretty if pretty else _canonical_dumps)(
+                data, sort_keys=sort_keys, indent=indent)
+        )
 
 
 def json_dumps_pretty(j, indent=2, sort_keys=True):