As far as I know, the 2010 A3 bumper is one piece.
The S3 bumper throughout is two piece - top half is the same as 2006 S-line cars but the lower half is different.
So in your pictures you can't put the diffuser from the black bumper on your car, because that bumper is one piece.
You need to change the whole lot.
If I were you, I'd keep your bumper and stick the S3 diffuser on the bottom, making it a S3 bumper.