Must admit the last time I spoke to Eric Bogle he stated that he did not mind his songs, particularly this one being changed. He was quite pleased that his song became popular even though many of the words and the title, had been changed. Many writers often change the words of their own songs, if they believe they need it, John Conolly and Punch and Judy man for example.
For those of us that attempt to write songs, how many are not happy with the first or even one hundred and first, attempt and change it. I tried singing the proper version of No Mans Land at a folk club only to be requested to sing the one that was made famous and everybody knew. Must admit I much prefer the original version.
Many songs are claimed to be Irish JHW, John Conolly's Fiddlers Green also appears in many song books as Irish traditional. Rose of Allendale is claimed both by the Iris and by the Scots and I believe it to be English (Northumberland)
Life is not simple and certainly folk music seems to have a life of its own, that's what I like about it. I certainly believe I have the right to change any song if I believe I should, and on the odd occasion do so. Fortunately for the rest of the world not many people hear me sing, just an unfortunate few in West and South Yorkshire.