I know from personal experience that a Hamish Imlac (not ancient, but tomorrow's folk) song, The Orange Juice Song*, was widely adapted to fit into a Norn' Ireland context back in the 60/70s.

It was written by Carl Macdougall. Imlach's contribution was to make it into a drunken macho anthem complete with ape grunts, in line with what a lot of male revival singers and groups of the time were doing (most famously the Clancy Brothers). There wasn't much change in the text (I think he left a verse or two out) but a substantial change in meaning.

"The Wild Rover" got an even more drastic transformation with even less textual change. Moralistic temperance song to drinking anthem.