Unfortunately you cant just change the ownerD value on a record from a update call if you would like to re assign a record. you have to use the AssignRequest Class to carry this out for you, i would assume that this is due to the security model setup on the CRM system.
Pasted a snippet, from some code i used on a plugin i was working on.
// Create the SecurityPrincipal object.
SecurityPrincipal assignee = new SecurityPrincipal();
assignee.Type = SecurityPrincipalType.User;
// PrincipalId is some known Guid belonging to the user or team that will own this record.
assignee.PrincipalId = new Guid("INSERT GUID OF THE USER YOU WANT TO ASSIGN THE RECORD TO");
// Create the target object for the request.
TargetOwnedAccount target = new TargetOwnedAccount();
// Set the properties of the target object.
// EntityId is some known Guid belonging to the account that is being assigned to the user.
target.EntityId = new Guid("INSERT GUID OF THE ACCOUNT YOU WOULD LIKE TO HAVE REASSIGNED");
// Create the request object.
AssignRequest assign = new AssignRequest();
// Set the properties of the request object.
assign.Assignee = assignee;
assign.Target = target;
// Execute the request.
AssignResponse assignResponse = (AssignResponse)service.Execute(assign);
Make sure you have set up the correct permissions on the users involved in the reassignment or this procedure will fail.
Enjoy
Jonathan
