Convert AC3 audio to mp3 in avi files without re-encoding the Video using FFMPEG

It seems that at some point Archos provided a firmware update that killed the ability to play my home videos (Xvid - MPEG-4/AC-3). I was offered to purchase the "Cinema Plug-in (DVD, MPEG-2, AC3)". Before that firmware update everything worked just fine, so it doesn't feel right to be forced to buy something to fix something that wasn't broken in the first place. If they were able to allow me to see AVI files with AC-3 audio encoding, in the first place, now coming back from it with the excuse you have to buy the codex doesn't sit right with me. If it was like that from the beginning, I wouldn't have any problem to pay my dues to any code-provider, but not after they provide a firmware-upgrade without warning with the changes and specifics.
So I'm not going to buy the plug-in, but rather convert my files to a format my Archos can play as is. My new recordings in HD, I convert with FFMPEG to Standard Definition files with H264/AAC encoding and they play just fine. Now I don't want to convert my old files to H264 as I don't want to reduce the video quality by re-encoding again. so I am trying to copy the video stream and re-encode only the audio stream in the same avi-container.

ffmpeg -threads 4 -i InputFile.avi -ar 48000 -ab 128k -acodec libmp3lame  -vcodec copy OutputFile.avi