diff --git a/update-customfield-semester.php b/update-customfield-semester.php
new file mode 100644
index 0000000..502b45d
--- /dev/null
+++ b/update-customfield-semester.php
@@ -0,0 +1,94 @@
+<?php
+// This file is part of Moodle - http://moodle.org/
+//
+// Moodle is free software: you can redistribute it and/or modify
+// it under the terms of the GNU General Public License as published by
+// the Free Software Foundation, either version 3 of the License, or
+// (at your option) any later version.
+//
+// Moodle is distributed in the hope that it will be useful,
+// but WITHOUT ANY WARRANTY; without even the implied warranty of
+// MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+// GNU General Public License for more details.
+//
+// You should have received a copy of the GNU General Public License
+// along with Moodle.  If not, see <http://www.gnu.org/licenses/>.
+
+/**
+ * Update semester custom field type.
+ *
+ * This script is used by the WWU Münster to update the already existing custom field from a select type to a
+ * customfield_semester type, and update the values for that field.
+ *
+ * Term independent courses get the value 1.
+ * All other courses get a value in the form of YYYYS, where YYYY is the year, in which the term begins, and S is either 0 for
+ * summer term or 1 for winterterm.
+ *
+ * @package    customfield_semester
+ * @copyright  2020 Justus Dieckmann WWU
+ * @license    http://www.gnu.org/copyleft/gpl.html GNU GPL v3 or later
+ */
+
+define('CLI_SCRIPT', 1);
+
+require_once(__DIR__ . '/../../../config.php');
+
+global $DB;
+
+$category = $DB->get_record('customfield_category', array('name' => 'Semester', 'component' => 'core_course', 'area' => 'course'));
+if (!$category) {
+    die('Customfield category does not exist!');
+}
+
+$field = $DB->get_record('customfield_field', array('name' => 'Semester', 'type' => 'select', 'categoryid' => $category->id));
+if (!$field) {
+    die('Customfield does not exist!');
+}
+
+// Update custom field to semester type
+$field->type = 'semester';
+$field->configdata = '{"required":"1","uniquevalues":"0","showmonthsintofuture":6,"defaultmonthsintofuture":3,"beginofsemesters":2007,"locked":"0","visibility":"2"}';
+$field->timemodified = time();
+$success = $DB->update_record('customfield_field', $field);
+if (!$success) {
+    die("customfield failure:" . $success);
+}
+
+// Update custom field data for every course.
+$courseids = $DB->get_fieldset_select('course', 'id', 'TRUE', array());
+foreach ($courseids as $courseid) {
+    $record = $DB->get_record('customfield_data', array(
+            'fieldid' => $field->id,
+            'instanceid' => $courseid
+    ));
+
+    if (!$record) {
+        echo "Error: course ($courseid) has no semester custom field<br>";
+        continue;
+    }
+
+    $value = (int) $record->intvalue;
+    if ($value >= 32) {
+        // Old semester id 32 stands for SS2022, is probably an error.
+        // The value will be converted anyway, but a warning will be printed.
+        echo "Warning: Value $value for course ($courseid) is far into future!<br>";
+    }
+
+    if ($value <= 1) {
+        // Semesterindependent
+        $record->intvalue = 1;
+    } else {
+        // Calculate new semesterid from old semesterid
+        // 2 => 20070
+        // 3 => 20071
+        // 4 => 20080
+        // etc.
+        $record->intvalue = ((int)($value / 2) + 2006) * 10 + $value % 2;
+    }
+    $record->value = $record->intvalue;
+    if (!$DB->update_record('customfield_data', $record)) {
+        echo "Error: Updating course ($courseid) failed!<br>";
+    }
+}
+
+echo "Finished!";
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