diff --git a/test/teos/unit/test_api.py b/test/teos/unit/test_api.py
index 46ba635..6d922cb 100644
--- a/test/teos/unit/test_api.py
+++ b/test/teos/unit/test_api.py
@@ -28,7 +28,14 @@ from test.teos.unit.conftest import (
 
 from common.blob import Blob
 from common.cryptographer import Cryptographer
-from common.constants import HTTP_OK, HTTP_NOT_FOUND, HTTP_BAD_REQUEST, HTTP_SERVICE_UNAVAILABLE, LOCATOR_LEN_BYTES
+from common.constants import (
+    HTTP_OK,
+    HTTP_NOT_FOUND,
+    HTTP_BAD_REQUEST,
+    HTTP_SERVICE_UNAVAILABLE,
+    LOCATOR_LEN_BYTES,
+    ENCRYPTED_BLOB_MAX_SIZE_HEX,
+)
 
 
 TEOS_API = "http://{}:{}".format(HOST, PORT)
@@ -201,7 +208,7 @@ def test_add_appointment_registered_no_free_slots(api, appointment):
 
 
 def test_add_appointment_registered_not_enough_free_slots(api, appointment):
-    # Empty the user slots
+    # Give some slots to the user
     api.gatekeeper.registered_users[client_pk_hex] = 1
 
     # Properly formatted appointment
@@ -217,7 +224,7 @@ def test_add_appointment_registered_not_enough_free_slots(api, appointment):
 
 
 def test_add_appointment_multiple_times_same_user(api, appointment, n=MULTIPLE_APPOINTMENTS):
-    # Multiple appointments with the same locator should be valid
+    # Multiple appointments with the same locator should be valid and counted as updates
     appointment_signature = Cryptographer.sign(appointment.serialize(), client_sk)
 
     # Simulate registering enough slots
@@ -251,6 +258,61 @@ def test_add_appointment_multiple_times_different_users(api, appointment, n=MULT
     assert len(set(api.watcher.locator_uuid_map[appointment.locator])) == n
 
 
+def test_add_appointment_update_same_size(api, appointment):
+    # Update an appointment by one of the same size and check that no additional slots are filled
+    api.gatekeeper.registered_users[client_pk_hex] = 1
+
+    appointment_signature = Cryptographer.sign(appointment.serialize(), client_sk)
+    r = add_appointment({"appointment": appointment.to_dict(), "signature": appointment_signature})
+    assert r.status_code == HTTP_OK and r.json().get("available_slots") == 0
+
+    # The user has no additional slots, but it should be able to update
+    # Let's just reverse the encrypted blob for example
+    appointment.encrypted_blob.data = appointment.encrypted_blob.data[::-1]
+    appointment_signature = Cryptographer.sign(appointment.serialize(), client_sk)
+    r = add_appointment({"appointment": appointment.to_dict(), "signature": appointment_signature})
+    assert r.status_code == HTTP_OK and r.json().get("available_slots") == 0
+
+
+def test_add_appointment_update_bigger(api, appointment):
+    # Update an appointment by one bigger, and check additional slots are filled
+    api.gatekeeper.registered_users[client_pk_hex] = 2
+
+    appointment_signature = Cryptographer.sign(appointment.serialize(), client_sk)
+    r = add_appointment({"appointment": appointment.to_dict(), "signature": appointment_signature})
+    assert r.status_code == HTTP_OK and r.json().get("available_slots") == 1
+
+    # The user has one slot, so it should be able to update as long as it only takes 1 additional slot
+    appointment.encrypted_blob.data = "A" * ENCRYPTED_BLOB_MAX_SIZE_HEX + "AA"
+    appointment_signature = Cryptographer.sign(appointment.serialize(), client_sk)
+    r = add_appointment({"appointment": appointment.to_dict(), "signature": appointment_signature})
+    assert r.status_code == HTTP_OK and r.json().get("available_slots") == 0
+
+    # Check that it'll fail if no enough slots are available
+    # Double the size from before
+    appointment.encrypted_blob.data = "AA" * ENCRYPTED_BLOB_MAX_SIZE_HEX + "AA"
+    appointment_signature = Cryptographer.sign(appointment.serialize(), client_sk)
+    r = add_appointment({"appointment": appointment.to_dict(), "signature": appointment_signature})
+    assert r.status_code == HTTP_BAD_REQUEST
+
+
+def test_add_appointment_update_smaller(api, appointment):
+    # Update an appointment by one bigger, and check slots are freed
+    api.gatekeeper.registered_users[client_pk_hex] = 2
+
+    # This should take 2 slots
+    appointment.encrypted_blob.data = "A" * ENCRYPTED_BLOB_MAX_SIZE_HEX + "AA"
+    appointment_signature = Cryptographer.sign(appointment.serialize(), client_sk)
+    r = add_appointment({"appointment": appointment.to_dict(), "signature": appointment_signature})
+    assert r.status_code == HTTP_OK and r.json().get("available_slots") == 0
+
+    # Let's update with one just small enough
+    appointment.encrypted_blob.data = "A" * (ENCRYPTED_BLOB_MAX_SIZE_HEX - 2)
+    appointment_signature = Cryptographer.sign(appointment.serialize(), client_sk)
+    r = add_appointment({"appointment": appointment.to_dict(), "signature": appointment_signature})
+    assert r.status_code == HTTP_OK and r.json().get("available_slots") == 1
+
+
 def test_get_appointment_no_json(api, appointment):
     r = requests.post(url=add_appointment_endpoint, data="random_message", timeout=5)
     assert r.status_code == HTTP_BAD_REQUEST