I would think they at least had the foresight to shoot “clean” background plates during TFA just in case they revisited this later.
I think you mean TLJ? Actually that’s interesting to consider. Back when Trevorrow was on IX he had Rian Johnson shoot a scene for IX while filming TLJ because it logistically made sense to do so. Never thought about it but definitely could have been an additional part of that flashback. Obviously no way to know at this point if that scene (whatever it actually is) will be used in JJ’s IX.