I interpret the below quote as: if there was a wav file and an mp3 file, they dumped the mp3 and created an mp4/aac from the wav (saving both the wav and the mp4/aac pair). Where there only was an mp3 file I assume they kept the mp3. Hence, not a lossy transcoding process.

> Some radio broadcasts were stored as mp3 and wav files, with accompanying checksum files. Other broadcasts were only stored as mp3. Before the re-archiving process began, it was decided to generate new MP4 playback files from the wav files to replace the varying qualities of the old mp3 files.