Automating MP3 conversion Options

[Blutarsky]

I have started ripping my CD library using FLAC format, as my main music archive.
Once done minor adjustments are performed: tags editing and cover art.
As the FLAC archive will have its MP3 counterpart for portable devices usage, I was wondering if there's a tool (command line would be fine as well) to automate generation of MP3s.
The tool should:
- scan the FLAC archive (folders & subfolders)
- generate MP3s only for new/changed FLACs (since last pass), creating the same folder/subfloder structure

Any idea?