diff --git a/chroot_version_hooks.d/45_64bit_convert_butterfly b/chroot_version_hooks.d/45_64bit_convert_butterfly
new file mode 100644
index 0000000..9125014
--- /dev/null
+++ b/chroot_version_hooks.d/45_64bit_convert_butterfly
@@ -0,0 +1,28 @@
+# Copyright (c) 2012 The Chromium OS Authors. All rights reserved.
+# Use of this source code is governed by a BSD-style license that can be
+# found in the LICENSE file.
+
+# This is meant to happen immediately after we update these boards
+# from 64bit to 32bit.  It moves the existing build root out of the
+# way and re-creates the build root.  The user must remove the copy
+# of the old build root manually if it is not needed.
+
+for board in butterfly; do
+  build="/build/${board}"
+  if [[ -d ${build} ]] ; then
+    case ${HOSTNAME:-$(hostname)} in
+    *.golo.chromium.org)
+      # Do not keep on buildbots.
+      sudo rm -rf "${build}"
+      ;;
+    *)
+      info "Moving 64bit ${board} to ${board}.64bit"
+      info "If you don't need it, please run "
+      info "sudo rm -rf ${build}.64bit"
+      sudo mv ${build}{,.64bit}
+      info "Running setup_board --board=${board}"
+      ~/trunk/src/scripts/setup_board --board=${board} --skip_chroot_upgrade
+      ;;
+    esac
+  fi
+done