how to convert all mp3 to m4a and save tags and covers?
The conversion &amp; the discard of the old mp3 will have to be done in 2 steps, so you need to make sure you've enough disk space to temporarily store double the amount of space the mp3s currently take up. If not, you'll have to do the conversion/discard in small batches, one album at a time.

Start in iTunes Prefs > General > Import Settings...
Set to AAC Encoder.

 

Then, back in iTunes main window, switch to My Music &amp; sort by Songs, which makes things easier to find.
If it doesn't show already even if you scroll the window to the right, then right-click the top of the columns in the main window &amp; add a column for Kind.

Click this column to sort by Kind 

The sort is simply alphabetical, so at the top will be AAC files, Apple Lossless etc., further down MPEG [mp3s] Below that may be other file types so make sure you don't accidentally select any of those.
Click the top MPEG file, scroll down &amp; shift/click the last one.

Right click one &amp; Select Create AAC version.

Wait until the process is finished - minute or hours depending on how many tunes...

You then need to find all the original mp3s that will still be in their original locations, now with an AAC file next to each one.

Navigate to your iTunes Music folder ~/Music/iTunes/iTunes Media/Music hit  Cmd ⌘   F  &amp; type mp3. The search will offer name matches: mp3 or kind is mp3 audio, pick kind.
Assuming you converted all your mp3s, anything showing in that search result can now be deleted. Simply select all &amp; Trash.

To be certain, relaunch iTunes &amp; scroll down the song list, making sure nothing has an exclamation mark ! at the left, which would indicate a missing file.

One you're sure, you can empty the trash.