A "folk song" or "folk tune"defined by its source and is still a folk song or tune if performed in a rock style (e.g. Whiskey in the Jar, House of the Rising Sun) but it is not "folk music" if it is performed in a non-participatory context".

So when classical singer Peter Pears sang 'The Lyke Wake Dirge' or Anthony Newley sang 'Strawberry fair' that continued to be 'folk'?
Likewise, if the London Symphony Orchestra played Beatles numbers, 'Yesterday' then could be considered a classical piece?
Try telling that to the punters on the South Bank.
In performance, folk music is much more than notes or words divorced from their style, origin or intention.
Jim Carroll