As per my knowledge when a LTE UE moves from one sector to another sector of same eNodeB in connected mode it considered as handover since source cell prepares RRC connection reconfiguration for target cell and send to UE. UE latches on target cell and confirms handover completion by sending RRC Connection Reconfiguration Complete to target Cell.
In this case there would not be any signalling towards core network since neither eNodeB nor core network elements changed.
It is eNodeB vendor how they have implemented these case. For UE it is a different serving cell than the previous one and it is getting data services at it is. I am not 100% sure on AS security aspect but believe security keys changes whenever there is a change on radio connection. Movement of UE from a cell to another cell considered a change in radio connection.
Please correct me if my understanding is completely wrong or partial wrong.