diff --git a/server/src/scripts/fix-new-thread-types.js b/server/src/scripts/fix-new-thread-types.js
new file mode 100644
index 000000000..e8b2e9974
--- /dev/null
+++ b/server/src/scripts/fix-new-thread-types.js
@@ -0,0 +1,119 @@
+// @flow
+
+import bots from 'lib/facts/bots.json';
+import { threadTypes, assertThreadType } from 'lib/types/thread-types';
+
+import { dbQuery, SQL } from '../database/database';
+import { createScriptViewer } from '../session/scripts';
+import { updateThread } from '../updaters/thread-updaters';
+import { main } from './utils';
+
+const batchSize = 10;
+const updateThreadOptions = { forceUpdateRoot: true };
+const threadObjectComparator = (a, b) => a.id - b.id;
+
+// When we introduced threadTypes.PERSONAL and threadTypes.PRIVATE, we made some
+// mistakes in how we converted existing threads into the new thread types:
+// (1) For both PRIVATE and PERSONAL, we didn't handle converting threads that
+//     had multiple roles properly. updateRoles was written to handle this, but
+//     we missed it and wrote some code that just converted all roles to the new
+//     role type instead of deleting extra roles and migrating those members
+//     over to the new single role.
+// (2) We allowed multiple threads per user to be converted into PRIVATE
+//     threads.
+// (3) We allowed threads with a parent to be converted into PRIVATE threads.
+// (4) We forgot to include EDIT_ENTRIES permissions for PRIVATE threads.
+async function fixNewThreadTypes() {
+  const fetchBrokenThreads = SQL`
+    SELECT t.id, t.type, t.parent_thread_id, MIN(m.user) AS user
+    FROM threads t
+    LEFT JOIN memberships m ON m.thread = t.id
+    WHERE t.type IN (${[threadTypes.PERSONAL, threadTypes.PRIVATE]})
+    GROUP BY t.id
+  `;
+  const [result] = await dbQuery(fetchBrokenThreads);
+
+  const forceUpdatePersonalThreadIDs = new Set();
+  const privateThreadsByUser = new Map();
+  for (const row of result) {
+    const id = row.id.toString();
+    const threadType = assertThreadType(row.type);
+    if (threadType === threadTypes.PERSONAL) {
+      forceUpdatePersonalThreadIDs.add(id);
+      continue;
+    }
+    const user = row.user.toString();
+    const parentThreadID = row.parent_thread_id
+      ? row.parent_thread_id.toString()
+      : null;
+    let userPrivateThreads = privateThreadsByUser.get(user);
+    if (!userPrivateThreads) {
+      userPrivateThreads = new Set();
+      privateThreadsByUser.set(user, userPrivateThreads);
+    }
+    userPrivateThreads.add({ id, parentThreadID });
+  }
+
+  const forceUpdatePrivateThreadIDs = new Set();
+  const unsetPrivateThreads = new Set();
+  for (const userPrivateThreads of privateThreadsByUser.values()) {
+    const sortedPrivateThreads = [...userPrivateThreads].sort(
+      threadObjectComparator,
+    );
+    while (sortedPrivateThreads.length > 0) {
+      const privateThread = sortedPrivateThreads.shift();
+      if (!privateThread.parentThreadID) {
+        forceUpdatePrivateThreadIDs.add(privateThread.id);
+        break;
+      }
+      unsetPrivateThreads.add(privateThread.id);
+    }
+    for (const privateThread of sortedPrivateThreads) {
+      unsetPrivateThreads.add(privateThread.id);
+    }
+  }
+
+  const updateThreadRequests = [];
+  for (const threadID of forceUpdatePersonalThreadIDs) {
+    updateThreadRequests.push({
+      threadID,
+      changes: {
+        type: threadTypes.PERSONAL,
+      },
+    });
+  }
+  for (const threadID of forceUpdatePrivateThreadIDs) {
+    updateThreadRequests.push({
+      threadID,
+      changes: {
+        type: threadTypes.PRIVATE,
+      },
+    });
+  }
+  for (const threadID of unsetPrivateThreads) {
+    updateThreadRequests.push({
+      threadID,
+      changes: {
+        type: threadTypes.CHAT_SECRET,
+        description: '',
+      },
+    });
+  }
+
+  const viewer = createScriptViewer(bots.squadbot.userID);
+  while (updateThreadRequests.length > 0) {
+    const batch = updateThreadRequests.splice(0, batchSize);
+    await Promise.all(
+      batch.map(async (updateThreadRequest) => {
+        console.log(`updating ${JSON.stringify(updateThreadRequest)}`);
+        return await updateThread(
+          viewer,
+          updateThreadRequest,
+          updateThreadOptions,
+        );
+      }),
+    );
+  }
+}
+
+main([fixNewThreadTypes]);