I think it will be very difficult to cause any problems using a syringe. When you top it up again, air bubbles will go up anyways. But, like Max said this is no change. You will actually bleed the system for long time to really change all fluid in all lines and calipers. The best way is a pressurized system working from the reservoir. Or the conventional way with two people. One to pump the pedal and one to open the bleed screws. The description in the Bentley with different procedures for ASC and ASC+T versions is ok. I usually need about 1.5 quarts of DOT 4 to be on the safe side. Pushing all pistons in the calipers back may be a good way to save some fluid because you can get more out of the reservoir. I hate the work of taking all pads out. Any good brake shop in Germany can test the fluid for water content. So you can find out if change is necessary.Regards Ekkehart