AC3 to AAC conversion

[Loomes]

What I have:
- A fine working streaming environment at home /w Synology NAS Server DS413j + Synology Apps on iPad, TV, PC etc.
- Loads of MP4 (h264 video + AC3 track) on my NAS, made from my DVD movies and DVD series boxes.

What I did:
- Converted DVDs to h264 video with 576p to 2000 kbps (fine for me) + AC3 track as it comes originally from the DVD (mostly 386 or even 640 kbps /w constant bitrate).

What I wanna do:
- Save even more space on my NAS by converting these AC3 tracks to AAC tracks while keeping the audio quality of the AC3 track.

My Question:
What is technically the right compression setting with NeroAACEnc to do conversion from AC3 to AAC? How is the technical background, what is the theoretical approach?

As far as I have read, AAC is way better than AC3 by saving space and quality in 2.0 and 5.1 audio tracks but I am unsure how to do it right and how far I can go re-compressing the AC3 tracks. I did some experiments with NeroAACEnc by doing "eac3to.exe mytrack.ac3 mytrack.m4a -quality=0.5 -2pass" meaning that the (already lossy) AC3 is deflated to WAV first, then compressed to M4A. I found out that a 348kbps constant AC3 will become a ~400kbps variable MP4 with slightly higher file size. Since AAC is the better compresser 0.5 cant be the right quality setting. The WAV originating from the AC3 is no better than the AC3, since AC3 is already lossy, so it makes no sense producing a bigger M4A.

What I want is to preserve the audio quality from the AC3, not loosing any more quality by doing the AAC compression. And of course I want the resulting M4A to be much smaller in file size than the AC3. Is there a trick or anything to do this properly?

Thanks in advance!