Convert MP3 to M4A
Drop an MP3 and download an .m4a file carrying AAC audio at 192 kbps. Sample rate and channel count are preserved, so a 48 kHz stereo source stays 48 kHz stereo.
Two lossy encoders in a row
The MP3 decoder feeds its output to the AAC encoder, which then makes its own decisions about what to discard. Both passes leave marks, and the second encoder cannot recover detail the first one dropped.
That is acceptable when a platform requires M4A. It is the wrong move when you still have the original WAV or FLAC, because encoding that source once to AAC produces a cleaner result at the same bitrate.
Container and compatibility
M4A is an MP4 container with a single audio track. Apple software, iOS, Android, and every current browser play it. Older car stereos and standalone MP3 players are where support runs out, so keep the MP3 when that hardware matters.