There are two obvious extremes with regard to derivative works. If the song is substantively dissimilar in form (for example, it might only borrow a motif which is modified, and be completely different in all other respects), then it may not necessarily be a derivative work (depending on how heavily modified the melody and chord progressions were). On the other hand, if the remixer only changes a few things (for example, the instrument and tempo), then it is clearly a derivative work and subject to the copyrights of the original work's copyright holder.