Maybe some misunderstanding or misinformation here.
Not much is set in stone. One or two fairly basic facts...FABs are a political initiative being pursued by the European Commission...and they will happen (by the end of next year. What it means and what might change is still being discussed.
The Commission sees FABs as a way to harmonise the ATM system and to reduce costs.
In theory all of the airspace in a FAB should be operated in the same way. Equally in theory it already is because of ICAO SARPs and so on - how much more same is wanted isn't clear.
In the short-term not much is likely to change, sector boundaries might change a bit and procedures might be aligned in some way but it seems, mainly, to be a paperwork exercise. In the medium-term things may get more interesting depending on a) how much political will there is to make big changes and b) whether any ANSPs, particularly the big players, see a way of making money from a FAB.