If it's an illegal file, redownload it (and hopefully get caught 😛 Serves you right).
If you own the file properly, then just re-rip it.
Surely there is no issue with re-ripping, unless it is a song recorded by a band/people you know (non-commercial).
It's better to leave it as an mp3, or the quality will go downhill. Why would you need it as wma?