Good thoughts. Here is a mediocre one to add: There are a couple of different ways to go.
Originally "Ol' 97" said, "It was on that grade that he lost his average," meaning average air pressure in the brake lines. Very, very few people would understand that, so I have no prob staying with "lost his air brakes," even though that isn't original.
On "Faded Coat of Blue," the chorus "should" end "when a robe of white is GIVEN for a faded ...." I changed that right off to "TRADED for a faded" which gives what my lit teacher Ms. Stutz would have called an internal feminine rhyme. I change a Dylan song that way too, I think it's "Just Like a Woman." It's just more effective that way.
There are indeed wrong words--"Among the Spanish brave" above is a great example. Mondegreens are wrong words. Another case of a garbled original is the Rev. Andy Jenkins' "Wreck of the Royal Palm." He's got that train racing down the curve at 40 mph, making time amid the drenching shower, when in fact Royal Palm was 'drifting' the siding when Ponce de Leon rolled through an improperly set switch. So I sing it, "Then coming round the curve, at forty miles an hour/Ponce de Leon was making time...." I also omit the verses of Engine 143 which name the wrong train crew, which is no great sacrifice given the fact that that ballad has a zillion verses and variants.
So, if have a good reason for the change (more poetic, more understandable, more correct) then change. Do whatever you do consciously, for your own reasons, and the two halves of your brain should be in sync.
PS. Isn't it, "In eighteen hundred and FIFTY-three???"