Probably depends on slot availability at LHR. I would guess that the move of DL1 from a roughly 2300 departure to an earlier time came about as part of the rejiggering of DL slots at LHR after MIA-LHR was scrubbed and ATL-LHR went to 18 weekly. I suppose if DL sees total loads on JFK-LHR drop because of lacking a post 2100 departure, they might see if they can get something for later arrival into LHR, perhaps by swapping with a JV partner flight from AMS/CDG/FCO/LIN.