Regardless of bitrate and sample size, converting from one lossy compression format to another will always lose quality.

AAC and MP3 use different algorithms to determine which parts of the sound can easily be removed to make the filesize smaller. If you run both compression schemes over the same data, you will lose both sets of sound.

Even though the files may not end up any smaller, some of the sound data will be degraded.

(Technically, even if you recompress an MP3 as MP3, you will probably lose some sound data, even if you go from 128 to 192, same as recompressing a JPEG will always lose some more of the image data.)

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And 99 times out of a hundred, the loss will be below the threshold of human detection and so is irrelevant.

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