Install the new Farm People and UI modules via an update hook on existing sites.

farm.install

@@ -535,6 +535,13 @@ function farm_update_7024(&$sandbox) {
   db_query('INSERT INTO {field_revision_field_farm_log_owner} (' . $select . ')');
 }
 
+/**
+ * Install the Farm People and Farm UI modules.
+ */
+function farm_update_7025(&$sandbox) {
+  _farm_update_enable_modules(array('farm_people', 'farm_ui'));
+}
+
 /**
  * Update helper function: enable modules.
  */