When a service is associated with a organisation set the free allowance to

the default free allowance for the organisation type.

The update/insert for the default free allowance is done in a separate
transaction. Updates to services need to happen in a transaction to
trigger the insert into the ServicesHistory table. For that reason the
call to set_default_free_allowance_for_service is done after the service
is updated.
I've added a try/except around the set_default_free_allowance_for_service call to ensure we still get the update to the service but get an exception log if the update to annual_billing fails. I believe it's important to preserve the update to the service in the unlikely event that the annual_billing upsert fails.

tests/app/dao/test_annual_billing_dao.py

@@ -91,3 +91,21 @@ def test_set_default_free_allowance_for_service_using_correct_year(sample_servic
         25000,
         2020
     )
+
+
+@freeze_time('2021-04-01 14:02:00')
+def test_set_default_free_allowance_for_service_updates_existing_year(sample_service):
+    set_default_free_allowance_for_service(service=sample_service, year_start=None)
+    annual_billing = AnnualBilling.query.all()
+    assert not sample_service.organisation_type
+    assert len(annual_billing) == 1
+    assert annual_billing[0].service_id == sample_service.id
+    assert annual_billing[0].free_sms_fragment_limit == 10000
+
+    sample_service.organisation_type = 'central'
+
+    set_default_free_allowance_for_service(service=sample_service, year_start=None)
+    annual_billing = AnnualBilling.query.all()
+    assert len(annual_billing) == 1
+    assert annual_billing[0].service_id == sample_service.id
+    assert annual_billing[0].free_sms_fragment_limit == 150000