I'm going off my own experience - after ripping into AAC+ vs MP3, the sound quality difference is dramatic - quite shockingly, in fact. 32KB generally came out better than 128KB MP3, using the Winamp AAC+ ripper. 24K generally sounds better than 96KB MP3, that I definitely can verify.
Re the re-ripping - I'm used to running live streams from the likes of SAM2/SAM4, and doing dynamic transcoding. I hadn't considered that it'd be running direct off the MP3s. And while transcoding the MP3s to AAC files would be an option, it'd lose quality of course.
The latest Shoutcast and Icecast both support AAC+, but I'm not sure which of the variants is being run (haven't looked into it).
Re-ripping would be a pain, but it'd also dramatically increase the capacity for listeners if the main streams were switched to AAC+.