... AAC itself is more efficient for the same bitrate than MP3 ...So yes, I suggest to keep the same bitrate. Increasing it might give an incorrect sign of higher quality and decreasing it could boost the artifacts from transcoding.
FYI:http://wiki.hydrogenaud.io/index.php?title=Transcoding#Lossy-to-lossy_transcodingWhy you want to do that? The video container format doesn't support mp3? Player compatibility? Are there any alternatives to avoid transcoding?