Once your tunes have been compressed, any "information" in the waveform which would have been kept had you done it at 256k originally is lost.

C'mon, if you could rip at 128 and get 256 quality, why would anyone ever rip at 256? As a logical extension, you'd be able to rip at 32k and still get CD quality, and we'd be able to sell 6Gb empegs as holding 4000 tracks :)

Hugo