While some articles such as the NI Travel News one seems to assume that by Belfast they mean BHD, everything else has stated just "Belfast" which could either be BHD or BFS, he has also not stated that it will be BA aircraft, just saying that the link would be retained.. so to me these are the 4 options.. there could be more but these are the ones i can think of
1. BA moves to BFS and alongside EI operates X amounts of flights per day, allowing an early arrival into BFS from LHR on BA aircraft
2. BA pulls out of BHD and EI increases flights from BFS under codeshare, however another aircraft would have to be found, unless a sun destination gets the chop or the 3rd base aircraft remains in Winter.
3. BA pulls out of BHD, EI puts an aircraft in BHD for the start of Winter when one is normally removed from BFS operating LHR from both BFS and BHD
4. Nothing changes at all and BA continues to fly BHD-LHR in their own aircraft