Oh yes, I agree it is ethical and often a very good thing to do. I do a lot of Irish ballads, and have done a lot of research into them. It's very common to come across several words that have changed from the original, usually making no difference to tthe song, and more often improving it, and as somebody else correcting it. As Pete Seeger sas in his book Rise Up Singing, it's an important part of the Folk process to keep the songs alive and add or remove things as long as it improves the song. And if it's good enough for Pete it's good enough for me.

Desi C
The Circle Folk Club
Coseley West Mid's