If you sing a cover song and change something about the track (whether it be lyrics, timing, key), your cover has now become a derivative work. Think of it as deriving from another piece of music and, therefore, become something new. You will ALWAYS need permission to do these.

The Magic of Song Writing
Like all great musicians, great songwriters are probably born, not made. No tutor, no book, no course, can make you write songs that embed themselves in the memory of a generation.

