Feel free to change it if you want. I generally object to pretending that traditional songs are the work of some particular artist who clearly did not write the work.
I agree. However, with Carter's songs we must be extra careful: AP copyrighted every song they recorded, and he "modified" some of them (extra lyrics, different tempo, etc.) He probably even wrote (or re-wrote) some of them from scratch.
Will the Circle Be Unbroken? is exemplary: the song was already very well-known before the Carters recorded it, yet their version has become a classic on its own. And he did make some alteration to the original, both lyrically and musically.
We should probably check every Carter Family song in the database and compare it with this: http://www.mudcat.org/thread.cfm?threadid=117018&messages=28