How to convert a batch of MP3's in Apple Music to AAC, then automatically delete the old MP3s?
There is a free app called Adapter that will do batch convert but I'm not sure about the deletes. Since it's free it's worth a try. If it doesn't do the deletes you can always sort the files by kind so that all mp3s will be listed together in the Finder window.  Adapter also converts images and movies as well as sound files.