Hypothetically, not a real world issue ... worst case, you have a 7 mm raised face (one for each flange), and a compressed gasket thickness of 3-4 mm (kammprofile). Keeping everything the same and changing to flat face, and assuming 1.5-2 mm (compressed) gasket thickness, you 'lose' some 15 mm.
The B16.5 bolt length is, for most cases, a bit too short if you need 2 - 3 threads after the nut. Nevertheless, I believe the length meets Code requirements for most, if not all, cases. So changing from a facing with metallic gasket to flat face will not impact Code requirement. If you, on the other hand, still have sufficient threads after the nut, is a different matter, although it's less important for flat face gaskets I presume, due to their low gasket seating stresses and therefore, bolt stresses.