I am pretty sure it wouldn't be illegal to do so to begin with (in the United States), so it wouldn't become illegal because you bought a copy legally.

It is specifically illegal to upload, share, or distribute copyrighted material. Downloading, streaming, or otherwise using copyrighted material for private use is not illegal. The reason why there were several high-profile cases where people were fined massive amounts of money for pirated songs is because they used torrenting software, which works by having all users upload and download peer-to-peer. By torrenting files, you are almost certainly distributing to dozens of other people who are also attempting to download that file, and have thus broken the law.

If you are in a location where it's illegal to download/stream pirated material, rather than just to distribute, you'd need to contact a lawyer to answer this question, because then you get into the very specific legal question of "is it illegal to manipulate a legal method of downloading music (streaming it from Youtube) and manipulate it in a way the content owner did not expect (ripping the audio to an MP3 file)."