BillD, and his Trader Joe Mexican Food problem, has finally defined the issue in a way that makes it more addressable--to wit:

The "Mexican Food" items that BillD talks about originally were food items from Mexico that TJ's simply packaged and sold to a wider market outside the culture of origin, and in that sense, were both "traditional" and "folk" foods that had been borrowed.

Over time, the recipes are modified by the borrowers to suit their own needs and tastes, which are different from the folk/ethnic culture where the food item originated. The question is, how much, if any, does the food change before it stops being Mexican?

In the same way, the Folk/Traditional music of certain British/American subcultures was collected, compiled, and introduced to wider audience outside the culture of origin(by Child,Sharp,tge Lomaxes, et al), and in that sense, it was both "Traditional" and "folk" music that had been borrowed.

Over time, the music was modified by the borrowers to suit their own needs and tastes, which differed from the folk/ethnic culture where the music originated. The question is, how much, if any, does the music change before it stops being "folk/traditional" music?

You can work it out from here...