diff --git a/UPGRADE.txt b/UPGRADE.txt
index 4cbcdffa75..bc5f9be212 100644
--- a/UPGRADE.txt
+++ b/UPGRADE.txt
@@ -501,6 +501,13 @@ API:
   res_agi, you will need to add ast_ prefixes to the functions that you call
   from those modules.
 
+ABI:
+
+* Many locking-related functions have had their ABI changed.  This may cause
+  external modules (including those provided by Asterisk-addons) to fail to
+  load with 'symbol not found' errors.  These modules will need to be
+  recompiled against a more current version of Asterisk.
+
 Formats:
 
 * format_wav: The GAIN preprocessor definition has been changed from 2 to 0