Actually as long as you refuse flipping, new units arise, at least this served as rule for my games since a long time (some betas ago). However, my last game with the byzantines featured no ottoman invasion on Europe, but instead, they spawned like 4 Armored Lancer above Nicaea after the flip, it was never so easy, when you are used to lots of Great Bombard spawning above Smyrna and under Hadrianopolis (not to say the dozens of units with them, Knights, Pikes and Muskets) in two different stacks, aiming at Constantinople (that only saw Battle in a few ocasions as of the Bulgarians - if you started at war - and any treacherous crusade, even less how better you make your expansion)
But I think now maybe this spawn is more accurate to the situation, and it was: Ottomans destroyed on their second turn; reason: They left the Capital with all units except a settler and one or two islamic missionaries. No difficult terrain between us (thanks this new Palm Tree feature), one unit conquest: Ottoman is destroyed, all units die. So this is a bug that still persists, it's nonsense why the AI do that. So after that: Flip, and then 4 Armored Lancer above Nicaea. More then 20 Cataphract at Constantinople and a few Knights. They could Attack, win and return to the not flipping zone, so the Ottomans were easily handled.