Just to clarify, you cannot get extra detail out of a 128 kbps file by upconverting it. Let me make a rough analogy. Imagine you have two buckets, the smaller one holding 128 ounces of water, the larger holding 256. The one labeled 128 is full with water, the one labeled 256 is empty. You then decide to pour all the water in the 128 bucket into the 256 bucket. However you can only fill half of the 256 bucket, because it's twice as large as the 128 bucket, wasting half of the bucket's water-carrying potential. Heck, you might have even lost a few drops of water in the process of pouring it into the larger bucket!
That's essentially what happens when you try to up-convert a music file. The bitrate tells you how much information a file can carry. Putting a small amount of information into a larger container doesn't actually give you any more information, and you might instead actually lose a bit of information in the processof trying to do so.
