Transcoding

Every time you encode with a Lossy encoder, the quality will decrease. There's no way to gain quality back even if you decode your 128kbps MP3 and encode it with 320kbps.

Transcoding to a different format is not recommended, unless absolutely necessary eg. for portable player use. It will always end up to a worse quality file than the source. People want to keep the new formats like Vorbis, MPC and AAC clean from the (possibly very bad quality) MP3 transcodings.