diff --git a/src/password_manager/encryption.py b/src/password_manager/encryption.py
index c309e52..9b02115 100644
--- a/src/password_manager/encryption.py
+++ b/src/password_manager/encryption.py
@@ -25,6 +25,8 @@ from typing import Optional
 import base64
 from cryptography.hazmat.primitives.ciphers.aead import AESGCM
 from cryptography.exceptions import InvalidTag
+from cryptography.fernet import Fernet, InvalidToken
+from cryptography.fernet import Fernet, InvalidToken
 from termcolor import colored
 from utils.file_lock import (
     exclusive_lock,
@@ -34,6 +36,16 @@ from utils.file_lock import (
 logger = logging.getLogger(__name__)
 
 
+def decrypt_legacy_fernet(encryption_key: bytes | str, payload: bytes) -> bytes:
+    """Decrypt *payload* using legacy Fernet."""
+    if isinstance(encryption_key, str):
+        key = encryption_key.encode()
+    else:
+        key = encryption_key
+    f = Fernet(key)
+    return f.decrypt(payload)
+
+
 class EncryptionManager:
     """
     EncryptionManager Class
@@ -55,6 +67,7 @@ class EncryptionManager:
         try:
             if isinstance(encryption_key, str):
                 encryption_key = encryption_key.encode()
+            self.key_b64 = encryption_key
             self.key = base64.urlsafe_b64decode(encryption_key)
             self.cipher = AESGCM(self.key)
             logger.debug(
@@ -304,16 +317,31 @@ class EncryptionManager:
             data = json.loads(json_content)
             logger.debug(f"JSON data loaded and decrypted from '{file_path}': {data}")
             return data
-        except json.JSONDecodeError as e:
-            logger.error(
-                f"Failed to decode JSON data from '{file_path}': {e}", exc_info=True
+        except (InvalidTag, json.JSONDecodeError):
+            logger.info(
+                f"AES-GCM decryption failed for '{file_path}', attempting legacy format"
             )
-            raise
-        except InvalidTag:
-            logger.error(
-                "Invalid encryption key or corrupted data while decrypting JSON data."
-            )
-            raise
+            with exclusive_lock(file_path) as fh:
+                fh.seek(0)
+                legacy_bytes = fh.read()
+            try:
+                legacy_plain = decrypt_legacy_fernet(self.key_b64, legacy_bytes)
+                data = json.loads(legacy_plain.decode("utf-8").strip())
+            except (InvalidToken, json.JSONDecodeError) as e:
+                logger.error(
+                    f"Legacy decryption failed for '{file_path}': {e}", exc_info=True
+                )
+                raise
+
+            legacy_path = file_path.with_suffix(file_path.suffix + ".fernet")
+            os.rename(file_path, legacy_path)
+            chk = file_path.parent / f"{file_path.stem}_checksum.txt"
+            if chk.exists():
+                chk.rename(chk.with_suffix(chk.suffix + ".fernet"))
+
+            self.save_json_data(data, relative_path)
+            self.update_checksum(relative_path)
+            return data
         except Exception as e:
             logger.error(
                 f"Failed to load JSON data from '{file_path}': {e}", exc_info=True
diff --git a/src/password_manager/vault.py b/src/password_manager/vault.py
index 78d8b99..e5fe002 100644
--- a/src/password_manager/vault.py
+++ b/src/password_manager/vault.py
@@ -30,6 +30,17 @@ class Vault:
     # ----- Password index helpers -----
     def load_index(self) -> dict:
         """Return decrypted password index data as a dict, applying migrations."""
+        legacy_file = self.fingerprint_dir / "seedpass_passwords_db.json.enc"
+        if legacy_file.exists() and not self.index_file.exists():
+            legacy_checksum = (
+                self.fingerprint_dir / "seedpass_passwords_db_checksum.txt"
+            )
+            legacy_file.rename(self.index_file)
+            if legacy_checksum.exists():
+                legacy_checksum.rename(
+                    self.fingerprint_dir / "seedpass_entries_db_checksum.txt"
+                )
+
         data = self.encryption_manager.load_json_data(self.index_file)
         from .migrations import apply_migrations, LATEST_VERSION
 
diff --git a/src/tests/test_legacy_migration.py b/src/tests/test_legacy_migration.py
new file mode 100644
index 0000000..a179cd9
--- /dev/null
+++ b/src/tests/test_legacy_migration.py
@@ -0,0 +1,43 @@
+import json
+import hashlib
+from pathlib import Path
+
+from helpers import create_vault, TEST_SEED, TEST_PASSWORD
+from utils.key_derivation import derive_index_key
+from cryptography.fernet import Fernet
+
+
+def test_legacy_index_migrates(tmp_path: Path):
+    vault, _ = create_vault(tmp_path, TEST_SEED, TEST_PASSWORD)
+
+    key = derive_index_key(TEST_SEED)
+    data = {
+        "schema_version": 4,
+        "entries": {
+            "0": {
+                "label": "a",
+                "length": 8,
+                "type": "password",
+                "kind": "password",
+                "notes": "",
+                "custom_fields": [],
+                "origin": "",
+                "tags": [],
+            }
+        },
+    }
+    enc = Fernet(key).encrypt(json.dumps(data).encode())
+    legacy_file = tmp_path / "seedpass_passwords_db.json.enc"
+    legacy_file.write_bytes(enc)
+    (tmp_path / "seedpass_passwords_db_checksum.txt").write_text(
+        hashlib.sha256(enc).hexdigest()
+    )
+
+    loaded = vault.load_index()
+    assert loaded == data
+
+    new_file = tmp_path / "seedpass_entries_db.json.enc"
+    assert new_file.exists()
+    assert not legacy_file.exists()
+    assert not (tmp_path / "seedpass_passwords_db_checksum.txt").exists()
+    assert (tmp_path / ("seedpass_entries_db.json.enc.fernet")).exists()