With AAC, I'm not sure how good the compression is.
Though converting n 128kbps AAC file to an 128bps MP3 file, when the AAC file has already converted it prior to the next conversion, you definietly won't get a boost in sound quality from one to the other since its already been converted the once into another format.
Increasing the bitrate now by the way wouldn't increase the sound quality either of course, because your more likely only to create extra noise, and a bigger size on the file, since its already been encoded into 128kbps.
But I doubt MP3 is any worse than AAC, so, more likely, you won't encounter any difference what so ever from swapping.
The only way you would, would be if the MP3 conversion was worse than AAC, in which case, you'd get a worse sound, but I doubt this, so it should just be a simple process.
If your not sure how to do it though, Itunes can. You just go into Edit, Preferences, and go to Advanced, and then the Importing tab.
Make it import songs in the MP3 format, at what ever bitrate you like, and then press to accept.
If you then right click a file, it should have "convert selection to MP3".
I hope this helps