diff --git a/cms/djangoapps/contentstore/rest_api/v2/views/downstreams.py b/cms/djangoapps/contentstore/rest_api/v2/views/downstreams.py
index cf76c8b206d4..5079698082be 100644
--- a/cms/djangoapps/contentstore/rest_api/v2/views/downstreams.py
+++ b/cms/djangoapps/contentstore/rest_api/v2/views/downstreams.py
@@ -4,7 +4,7 @@
 API paths (We will move these into proper api_doc_tools annotations soon
 https://github.com/openedx/edx-platform/issues/35653):
 
-  /api/v2/contentstore/downstreams/{usage_key_string}
+  /api/contentstore/v2/downstreams/{usage_key_string}
 
     GET: Inspect a single downstream block's link to upstream content.
       200: Upstream link details successfully fetched. Returns UpstreamLink (may contain an error_message).
@@ -26,7 +26,7 @@
       400: Content at upstream_ref does not support syncing.
       404: Downstream block not found or user lacks permission to edit it.
 
-  /api/v2/contentstore/downstreams/{usage_key_string}/sync
+  /api/contentstore/v2/downstreams/{usage_key_string}/sync
 
     POST: Sync a downstream block with upstream content.
       200: Downstream block successfully synced with upstream content.
@@ -41,8 +41,8 @@
       404: Downstream block not found or user lacks permission to edit it.
 
   # NOT YET IMPLEMENTED -- Will be needed for full Libraries Relaunch in ~Teak.
-  /api/v2/contentstore/downstreams
-  /api/v2/contentstore/downstreams?course_id=course-v1:A+B+C&ready_to_sync=true
+  /api/contentstore/v2/downstreams
+  /api/contentstore/v2/downstreams?course_id=course-v1:A+B+C&ready_to_sync=true
       GET: List downstream blocks that can be synced, filterable by course or sync-readiness.
         200: A paginated list of applicable & accessible downstream blocks. Entries are UpstreamLinks.
 
diff --git a/cms/lib/xblock/upstream_sync.py b/cms/lib/xblock/upstream_sync.py
index 8dbe672a5d10..62a4bd7f4aa6 100644
--- a/cms/lib/xblock/upstream_sync.py
+++ b/cms/lib/xblock/upstream_sync.py
@@ -223,10 +223,22 @@ def _load_upstream_link_and_block(downstream: XBlock, user: AbstractUser) -> tup
 
 def _update_customizable_fields(*, upstream: XBlock, downstream: XBlock, only_fetch: bool) -> None:
     """
-    For each customizable field `downstream.blah`:
+    For each customizable field:
+    * Save the upstream value to a hidden field on the downstream ("FETCH").
+    * If `not only_fetch`, and if the field *isn't* customized on the downstream, then:
+      * Update it the downstream field's value from the upstream field ("SYNC").
 
-    1. Fetch `upstream.blah` and save it in `downstream.upstream_blah`.
-    2. If not only_fetch, and if `downstream.blah` isn't customized, update it from `upstream.blah`.
+    Concrete example: Imagine `lib_problem` is our upstream and `course_problem` is our downstream.
+
+     * Say that the customizable fields are [display_name, max_attempts].
+
+     * Set `course_problem.upstream_display_name = lib_problem.display_name` ("fetch").
+     * If `not only_fetch`, and `course_problem.display_name` wasn't customized, then:
+       * Set `course_problem.display_name = lib_problem.display_name` ("sync").
+
+     * Set `course_problem.upstream_max_attempts = lib_problem.max_attempts` ("fetch").
+     * If `not only_fetch`, and `course_problem.max_attempts` wasn't customized, then:
+       * Set `course_problem.max_attempts = lib_problem.max_attempts` ("sync").
     """
     syncable_field_names = _get_synchronizable_fields(upstream, downstream)
 
@@ -235,7 +247,7 @@ def _update_customizable_fields(*, upstream: XBlock, downstream: XBlock, only_fe
         if field_name not in syncable_field_names:
             continue
 
-        # Fetch the upstream's value and save it on the downstream (ie, `downstream.upstream_$FIELD`).
+        # FETCH the upstream's value and save it on the downstream (ie, `downstream.upstream_$FIELD`).
         old_upstream_value = getattr(downstream, fetch_field_name)
         new_upstream_value = getattr(upstream, field_name)
         setattr(downstream, fetch_field_name, new_upstream_value)
@@ -257,7 +269,7 @@ def _update_customizable_fields(*, upstream: XBlock, downstream: XBlock, only_fe
         if old_upstream_value and downstream_value != old_upstream_value:
             continue  # Field has been customized. Don't touch it. Move on.
 
-        # Field isn't customized -- update it!
+        # Field isn't customized -- SYNC it!
         setattr(downstream, field_name, new_upstream_value)
 
 
@@ -295,10 +307,7 @@ def decline_sync(downstream: XBlock) -> None:
 
     If `downstream` lacks a valid+supported upstream link, this raises an UpstreamLinkException.
     """
-    # Try to load the upstream.
     upstream_link = UpstreamLink.get_for_block(downstream)  # Can raise UpstreamLinkException
-    if not upstream_link:
-        return  # No upstream -> nothing to sync.
     downstream.upstream_version_declined = upstream_link.version_available