Altered means fully changing/removing the music if you don't have the rights for it in the first place. If it is coincident match, you just dispute it. Changing the pitch or something to trick the ContentID system is like asking for trouble. If a human notices the similarity they will issue direct DMCA that will result in a strike for your channel and some features removed. I had once been DMCAd for all the wrong reasons (that was retracted later), but it was not good feeling having my channel limited.
Acquiring license means contacting the copyright holders/managers and buying/negotiating a deal about the music. This is expensive for popular songs. For not so popular a license starts from 1000 USD. For really cheap songs you can get them at 2.99 USD or even 1 USD from the royalty free sites like AudioJungle.