diff --git a/authens/backends.py b/authens/backends.py
index fcfd57a..92df47e 100644
--- a/authens/backends.py
+++ b/authens/backends.py
@@ -1,7 +1,7 @@
 from django.contrib.auth import get_user_model
 from django.db import transaction
 
-from authens.models import CASAccount
+from authens.models import CASAccount, OldCASAccount
 from authens.utils import get_cas_client
 
 UserModel = get_user_model()
@@ -101,8 +101,8 @@ class ENSCASBackend:
         """Handles account retrieval, creation and invalidation as described above.
 
         - If no CAS account exists, create one;
-        - If a CAS account exists, but with the wrong entrance year, remove it and
-          create a new one;
+        - If a CAS account exists, but with the wrong entrance year, deprecate it
+        into an OldCASAccount;
         - If a matching CAS account exists, retrieve it.
         """
 
@@ -113,6 +113,11 @@ class ENSCASBackend:
             try:
                 user = UserModel.objects.get(cas_account__cas_login=cas_login)
                 if user.cas_account.entrance_year != entrance_year:
+                    OldCASAccount.objects.create(
+                        user=user,
+                        entrance_year=user.cas_account.entrance_year,
+                        cas_login=cas_login,
+                    )
                     user.cas_account.delete()
                     user = None
             except UserModel.DoesNotExist:
@@ -132,3 +137,27 @@ class ENSCASBackend:
             return UserModel.objects.get(pk=user_id)
         except UserModel.DoesNotExist:
             return None
+
+
+class OldCASBackend:
+    def authenticate(self, request, cas_login=None, password=None, entrance_year=None):
+        if cas_login is None or password is None or entrance_year is None:
+            return
+
+        try:
+            old_cas_acc = OldCASAccount.objects.get(
+                cas_login=cas_login, entrance_year=entrance_year
+            )
+            user = old_cas_acc.user
+        except OldCASAccount.DoesNotExist:
+            # As in Django's ModelBackend, we run the password hasher once
+            # to avoid timing attacks
+            UserModel().set_password(password)
+        else:
+            if user.check_password(password) and self.user_can_authenticate(user):
+                return user
+
+    def user_can_authenticate(self, user):
+        # Taken from Django's ModelBackend
+        is_active = getattr(user, "is_active", None)
+        return is_active or is_active is None