I started this subreddit with big ideas, but I never really got it off the ground. This falls under my usual motto, "I have a great idea, but I don't want to put a lot of effort into it."
That being said, you could write a script to convert all of your music and I could help you with that. I see it as a bit of a fun challenge, actually. What operating system are you running?
Edit: I'm also going to include this link. There are a ton of free LAME tools you can use on any operating system to transcode your 256kb songs to 192kb. Just make sure you are doing 192 VBR (variable bitrate) and not constant. This will allow the bitrate to drop lower when there's less data required and go a bit higher when there's more data required. http://lame.sourceforge.net/links.php#OpenSource
You are going to find that the variable settings in LAME encoding (or transcoding in this case) drop off significantly after -V4 and then pretty much plummet after -V5. http://wiki.hydrogenaudio.org/index.php?title=File:Lame-chart-2.png I would probably not go lower than -V4. This will be a setting in whatever program you ultimately use to convert your media.
If you can't get one of those apps do it all in bulk, let me know what operating system and I'll give you a batch file you can run from the command line to do it.
Edit2: Most scene releases are in -V2. Most audiophiles who tolerate MP3 use -V0.
Edit3: Make a backup of your music before you do this. If the conversion doesn't go well, you could destroy your audio library.