diff --git a/changelog.d/6359.bugfix b/changelog.d/6359.bugfix
new file mode 100644
index 000000000..22bf5f642
--- /dev/null
+++ b/changelog.d/6359.bugfix
@@ -0,0 +1 @@
+Fix bug where upgrading a guest account to a full user would fail when account validity is enabled.
\ No newline at end of file
diff --git a/synapse/storage/_base.py b/synapse/storage/_base.py
index 1a2b7ebe2..ab596fa68 100644
--- a/synapse/storage/_base.py
+++ b/synapse/storage/_base.py
@@ -361,14 +361,11 @@ class SQLBaseStore(object):
                 expiration_ts,
             )
 
-        self._simple_insert_txn(
+        self._simple_upsert_txn(
             txn,
             "account_validity",
-            values={
-                "user_id": user_id,
-                "expiration_ts_ms": expiration_ts,
-                "email_sent": False,
-            },
+            keyvalues={"user_id": user_id},
+            values={"expiration_ts_ms": expiration_ts, "email_sent": False},
         )
 
     def start_profiling(self):