If you lower the bit rate to 128 from the DRM free songs which are 256 then the quality of your music is going to decline. You stated you listen at work to the louder version which would be the 256 drm free songs.
I am personally trying to raise my bit rate, not lower it. Everything I do now is 256 VBR which for the most part gives a higher bitrate then the 256 norm.
To answer your question you can always down grade a song to the 128. But once done, you will loose the quality and it can not be reversed. If you go into settings and the advance tab. Change to .mp3 128 bitrate. Then go into your music library and find those 256 songs and right click on advance and select convert song to mp3. This will only work on the DRM Free songs or those that you have ripped.
Still don't understand why you want to lower your quality, but there it is.
Good Luck