I suppose there might be a way to do this, but surely, after a while, you'll be spending a lot of time determining whether a song is already at 128Kbps, or can you save further space by changing it to 128K?
Have you thought about Syncing only selected Playlists to your iPod?
In other words, instead of Syncing all your music to the iPod, use the Sync only selected Playlists option.This would then enable you to put music into various Playlists and Sync only those. Fancy a change? Remove one Playlist (with music choice A in it) and replace it with another Playlist with music choice B in it.
You could create a Smart Playlist that only has songs that have a bit rate that is GREATER THAN 128kbps. Then you'll know which ones to convert. Then you could have another Smart Playlist that has songs that the bit rate is LESS THAN 140kbps (or some margin you choose). This would be the playlist that you could sync to your iPod.