update originatingleadid

I'm trying to update an account with an originating lead (this is a data migration deal so I am not in normal channels.)  I've tried this and nothing happens.  I'm wondering if it is possible or if I'm neglecting something:

        static void acctGetOrigLead(IOrganizationService service, Guid acctid, Guid lead) {
            EntityReference origLead = new EntityReference("lead", lead);
            Entity acct = new Entity();
            acct.LogicalName = "account";
            acct.Id = acctid;
            acct.SetAttributeValue<EntityReference>("originatingleadid", origLead);
//            acct.Attributes.Add("originatingleadid", origLead);
            service.Update(acct);
        }

No errors.  It just runs to the end and yields no results.

Thanks