My response to the original question "is it permissible....?":

There are many who will proceed to make changes, constructive and otherwise, without asking permission. And, for those who feel a need for permission, who would they ask?

The word "sacrosanctity" immediately occurred to me when reading some of the more hard-core traditionalist responses. I was pleasantly surprised to see one of the contributors with whom I most wholeheartedly disagree to use that very word:

"I hold the traditional singers and the songs they sang to be sacrosanct."

Admirable indeed. However, in reality (I would argue), what you are holding sacrosanct are versions of traditional songs as transcribed/recorded by collectors. The preservation of a particular version of a given song, as opposed to other interpretations, is an absolute accident of history ~ if Child, or Sharp, or Lomax, or whoever, had wandered into the next valley over, or found his/her way to a different fishing villiage, it's very likely that another variant of the same song (or a similar song) would have become that song's sacred untouchable text. Silly!

Also, we need to keep in mind that our forebearers the folksong collectors were urban educated types with their own preconceptions and prejudices. When they ventured into isolated rural communities, to confront anachronistic and relatively alien subcultures, they were very likely to misunderstand and misinterpret at least a few subtleties of the local idiom and customs.

(I have no doubt that this disconnect often held sway when white northeastern American academics ventured into the very separate world of the early-20th-century African-American south. By extention, I feel safe in assuming that similar problems could have existed when Oxford/Cambridge types visited the more isolated folk cultres of the British Isles.)

Plus which, of course, the local folks may have felt an awareness that they were being perceived as unsophisticated oddities, and might well have deliberately "put on" their interrogators. (I believe the British expression is "taking the piss.") How hilarious that today's sacred untouchable text of some ballad or another might still contain some "primitive" person's idea of a good joke played upon a pretentious visitor to his/her village!