diff --git a/Modules/ExternalProject.cmake b/Modules/ExternalProject.cmake
index 52d41eb19..74e8ae27f 100644
--- a/Modules/ExternalProject.cmake
+++ b/Modules/ExternalProject.cmake
@@ -1456,6 +1456,15 @@ function(ExternalProject_Add_Step name step)
   if(always)
     set_property(SOURCE ${stamp_file} PROPERTY SYMBOLIC 1)
     set(touch)
+    # Remove any existing stamp in case the option changed in an existing tree.
+    if(CMAKE_CONFIGURATION_TYPES)
+      foreach(cfg ${CMAKE_CONFIGURATION_TYPES})
+        string(REPLACE "/${CMAKE_CFG_INTDIR}" "/${cfg}" stamp_file_config "${stamp_file}")
+        file(REMOVE ${stamp_file_config})
+      endforeach()
+    else()
+      file(REMOVE ${stamp_file})
+    endif()
   else()
     set(touch ${CMAKE_COMMAND} -E touch ${stamp_file})
   endif()