Converting from one lossy codec another isn't generally recommended, plus you aren't likely to save that much disk space by converting to AAC.

10 GB is actually pretty small for a local music collection, quite honestly. If I were you, I would try to expand your storage capacity instead of wasting time, and potentially audio quality, by transcoding.

Edit: I agree with other commenters, that it's a bad idea to convert from one lossy codec to another one! If you want to do it anyway (and your files are at least encoded with high bitrates >192k), I'd recommend this:

The best lossy audio codec by far is opus (best perceived quality vs. small file size), which also has the benefit that it's free and has got a great open source reference implementation that is also integrated in ffmpeg. So the conversion can be done with ffmpeg. I would personally use fd-find for multithreaded batch processing (using the -x option).

If possible, only convert the wav files to AAC and keep all lossy files as they are.

On Windows, nothing beats foobar for playback, tagging, and conversion support. I use Deadbeef which is like the Foobar of Linux. It has a similar user interface and a playlist format conversion tool as well. VLC also converted audio if I remember correctly?

That would be a pretty basic bash script, but as others have said, really not recommended.

Don't do it, but if you decide to do it anyway, use DbPoweramp

If you are a Mac users (which I am assuming yes based on your preference toward AAC), the program Pine Player does an excellent job with batch conversion. Otherwise FFMPEG is probably acceptable as well.

You can use ffmpeg command line with some loop (for) tricks. If you want to save some space without re converting them, use .flac or .mka container with -copy flags