If you wanted a hope of getting on the earlier flight, then I would consider an Avios booking if available.
BA won’t book you under MCT. I’m not sure how your bag would be affected as you’ll be a no show from your EI flight.
You’ve not been downgraded. There is no J on EI on this route.
With the terminal change and security, you won”t have much more than 3h to kill, if on time. Just relax and enjoy the airport