diff --git a/src/euphorie/upgrade/deployment/v1/20240423105626_clean_up_last_modifier_id/__init__.py b/src/euphorie/upgrade/deployment/v1/20240423105626_clean_up_last_modifier_id/__init__.py
new file mode 100644
index 000000000..e69de29bb
diff --git a/src/euphorie/upgrade/deployment/v1/20240423105626_clean_up_last_modifier_id/upgrade.py b/src/euphorie/upgrade/deployment/v1/20240423105626_clean_up_last_modifier_id/upgrade.py
new file mode 100644
index 000000000..d5d7ef1b1
--- /dev/null
+++ b/src/euphorie/upgrade/deployment/v1/20240423105626_clean_up_last_modifier_id/upgrade.py
@@ -0,0 +1,33 @@
+from euphorie.client.model import Account
+from euphorie.client.model import Session
+from euphorie.client.model import SurveySession
+from ftw.upgrade import UpgradeStep
+
+import logging
+
+
+logger = logging.getLogger(__name__)
+
+
+class CleanUpLastModifierId(UpgradeStep):
+    """Clean up last_modifier_id."""
+
+    def __call__(self):
+        session = Session()
+        obsolete_guest_users = (
+            session.query(Account, SurveySession)
+            .filter(Account.id == SurveySession.last_modifier_id)
+            .filter(Account.id != SurveySession.account_id)
+            .filter(Account.account_type == "guest")
+        )
+        for guest_user, assessment in obsolete_guest_users:
+            assessment.last_modifier_id = assessment.account_id
+            logger.info("Updated session %s", assessment.id)
+            num_assessments = (
+                session.query(SurveySession)
+                .filter(SurveySession.account_id == guest_user.id)
+                .count()
+            )
+            if num_assessments == 0:
+                session.delete(guest_user)
+                logger.info("Deleted user %s", guest_user.id)