I believe it is that you go back to 1 level before you specialised. So if you were a 3 you go back to 4. But say you were a 2b before specialising to go back to reffing you would demote to level 3 in stead.
That would make sense for legacy referees. However promotion to 2B now automatically pushes you off of lining no matter how many years you have been a 3.