Yep, Audacity or just LAME (with a front end if you're not happy with the command line) should do the trick.
Bare in mind though, converting a lossy format to another lossy format can degrade the quality quite badly. So if you convert a 192kbps mp3 to a 128kbps mp3, it'll sound worse than a 128kbps mp3 ripped straight from cd.
Hope that made sense. Suppose it won't matter just for web stuff though.