Why MP3 to AAC, for this MP3 file
Apple devices and some streaming apps prefer AAC to MP3.
The AAC sound you will actually get
The MP3 is re-encoded as AAC on your device. At a similar bitrate AAC can sound a little cleaner, but it cannot put back what the MP3 already removed.
Keep the MP3 if that is your only copy. This is another lossy generation, not a studio master.
Why MP3 to AAC is its own job
MP3 to AAC sits on this site because MP3 and AAC are not interchangeable, even when they look similar in a file list. Apple devices and some streaming apps prefer AAC to MP3. Podcast hosts, cars, WhatsApp, and old speakers expect MP3. Apple workflows and some streaming uploads prefer AAC.
The MP3 is re-encoded as AAC on your device. At a similar bitrate AAC can sound a little cleaner, but it cannot put back what the MP3 already removed.
The MP3 file you already have
MP3 is the audio file that plays everywhere. It is lossy. Music is often 192 to 320 kbps. Speech can be lower. Each MP3 save throws away a little more. Turning MP3 into WAV or FLAC does not bring the lost detail back.
Keep WAV or FLAC if you might edit again. Keep the MP3 if it is your only copy.
What a AAC result is for
The download has to be AAC, because that is the format the next step named. Apple workflows and some streaming uploads prefer AAC. It is a good share format when the player accepts it.
If the player already plays MP3, you do not need AAC.
How to tell that MP3 to AAC worked
MP3 to AAC is worth using only when AAC is what the next step actually named. Apple devices and some streaming apps prefer AAC to MP3. What you get from MP3 to AAC: The MP3 is re-encoded as AAC on your device. At a similar bitrate AAC can sound a little cleaner, but it cannot put back what the MP3 already removed. Before you trust it: Keep the MP3 if that is your only copy. This is another lossy generation, not a studio master. The file stays on your device. The first run downloads a converter, so keep this tab open. A phone runs out of memory on long files. A computer is the safer place for a big clip. A second pass rarely helps. If the first AAC file is rejected, read the error from the program that rejected it. Another conversion of a bad result usually makes the file worse, not more compatible. When you are done, you should still have the MP3 original and one AAC download that you have opened. If those two are the same kind of failure, this was the wrong tool and the page above is the place that says so. Audio conversions are one-way once the format is lossy. MP3 to AAC can change the container or the codec so a player opens the file. It cannot make a muffled take clearer, and it cannot turn an MP3 into a studio master by calling the result WAV or FLAC. Finish the edit on the best copy you have, then make the share file once.
Here is the decision for MP3 to AAC. You start with MP3 and you stop when a real program accepts AAC. Compatibility is the reason to make an MP3, not an improvement in sound. Converting from MP3 to AAC cannot restore what the MP3 removed. It is another lossy generation. If those two sentences describe a tradeoff you do not want, do not convert. Renaming the extension by hand will not do this job. Running the tool twice on its own output will not do it either.
Keep the MP3 if that is your only copy. This is another lossy generation, not a studio master.
A related step, if you still need one, is AAC to MP3.
Other jobs start from the homepage, where each tool is named for the conversion it actually does.