You could argue for a folk song to be changed by different hands it has to be fundamentally 'simple' in some way. That doesn't mean short or uncomplicated but that the central motifs have currency that can be adapted. If a song utilises the particular at the expense of the general, say for instance it's about a given historical character with a chronolgical build up, a tight thyme and well known tune it may be an old song but has no transferable currency that can make it a folk song.

In folklore emblematic variables are one of the defining traits.