Guid accountId = new Guid("90F8889F-EB95-E781-8417-000C44420CBC");
Guid newOwnerId = new Guid("A8AA28B4-9015-DF11-8062-000E0CA08051");
AssignRequest assignRequest = new AssignRequest
{
Assignee = new EntityReference("systemuser", newOwnerId),
Target = new EntityReference("account", accountId)
};
serviceProxy.Execute(assignRequest);
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