Aznev

hi If I have a mp3 file at 128 kbps and I convert it at 128 kbps again, does it loss quality? or its just like making a copy of it? I have a large mp3 collection, but at diferent bitrates. with switch I could just convert them all but I don't know if the ones at 128 kbps will be afected. Thaks for your answer.