Fix Block Transformer version checking so cache can be pre-populated

This change requires Block Transformer schema changes to be backwards
compatible for older Block Transformer code versions - at least 1
version back to allow smooth deployments in a multi-version setting.

openedx/core/lib/block_structure/tests/test_manager.py

@@ -169,8 +169,10 @@ class TestBlockStructureManager(TestCase, ChildrenMapTestMixin):
 
     def test_get_collected_outdated_data(self):
         self.collect_and_verify(expect_modulestore_called=True, expect_cache_updated=True)
-        TestTransformer1.VERSION += 1
+        TestTransformer1.VERSION += 1  # transformer code requires new schema version
         self.collect_and_verify(expect_modulestore_called=True, expect_cache_updated=True)
+        TestTransformer1.VERSION -= 1  # old transformer code works with new schema version
+        self.collect_and_verify(expect_modulestore_called=False, expect_cache_updated=False)
         self.assertEquals(TestTransformer1.collect_call_count, 2)
 
     def test_get_collected_version_update(self):

openedx/core/lib/block_structure/transformers.py

@@ -90,7 +90,7 @@ class BlockStructureTransformers(object):
         outdated_transformers = []
         for transformer in TransformerRegistry.get_registered_transformers():
             version_in_block_structure = block_structure._get_transformer_data_version(transformer)  # pylint: disable=protected-access
-            if transformer.VERSION != version_in_block_structure:
+            if transformer.VERSION > version_in_block_structure:
                 outdated_transformers.append(transformer)
 
         if outdated_transformers: