diff --git a/src/subscriptions/client.py b/src/subscriptions/client.py
index cab5248..3e851ac 100644
--- a/src/subscriptions/client.py
+++ b/src/subscriptions/client.py
@@ -155,3 +155,13 @@ def get_payment_info_url(sub_handle: str, accept_url: str, cancel_url: str) -> s
 
 def get_plan(plan_handle: str) -> dict:
     return _call("GET", f"/v1/plan/{plan_handle}")
+
+
+def change_subscription(
+    sub_handle: str, plan_handle: str, timing: str = "renewal"
+) -> dict:
+    return _call(
+        "PUT",
+        f"/v1/subscription/{sub_handle}",
+        json={"timing": timing, "plan": plan_handle},
+    )
diff --git a/tests/subscriptions/test_client.py b/tests/subscriptions/test_client.py
index e86d8c9..afd8ef0 100644
--- a/tests/subscriptions/test_client.py
+++ b/tests/subscriptions/test_client.py
@@ -131,3 +131,18 @@ def test_get_payment_info_url_merges_existing_query_string(fake_httpx):
     assert "locale=fr" in url
     assert "accept_url=https%3A%2F%2Fapp%2Fok" in url
     assert "cancel_url=https%3A%2F%2Fapp%2Fko" in url
+
+
+def test_change_subscription_sends_timing_and_plan(fake_httpx):
+    fake_httpx["queue"].append(fake_httpx["Response"](200, {"handle": "abo-1-1"}))
+    client.change_subscription("abo-1-1", "plan_soutien", timing="renewal")
+    call = fake_httpx["calls"][0]
+    assert call["method"] == "PUT"
+    assert call["url"] == "https://api.test/v1/subscription/abo-1-1"
+    assert call["json"] == {"timing": "renewal", "plan": "plan_soutien"}
+
+
+def test_change_subscription_immediate_timing(fake_httpx):
+    fake_httpx["queue"].append(fake_httpx["Response"](200, {"handle": "abo-1-2"}))
+    client.change_subscription("abo-1-2", "plan_simple", timing="immediate")
+    assert fake_httpx["calls"][0]["json"]["timing"] == "immediate"