How can I change the bit rate of MP3s in iTunes 11?
Although quality will be lost when converting the bit rate to 128kbps, one option is to use a combination of a smart playlist in iTunes and Max.

1) Either back up music library or specify a different output directory in Max, and then re-import files to iTunes.

2) In iTunes, create a smart playlist where the bit rate is greater than 128 kbps for example.

3) Select the files from the playlist and drag them to the conversion window in Max. Preferences in Max should be setup so that the output files will be stored in the same location as input files if you want the originals replaced. There is also an option checkbox to "Overwrite existing files" and "Delete source files after encoding". This will take care of the re-naming issue. There is also an option to add output files to the iTunes library under the iTunes 'tab'.

Depending on the size of your library, it may be necessary to convert the mp3 files in chunks.