my 10p worth in relation to the original question - I'd agree with others here that key mainly affects instrumentation and vocal ranges so it varies from one person or band to another and that is what is most important. As a member of a band singing (some of the time) in close unaccompanied harmony I'm constatntly amazed by how moving something by as little as a semitone seems to affect our mutual vocal comfort zone.

as far as collecting from traditional performers is concerned, I struggle to believe that they weren't affected by the same stuff as we are now - so would have picked the key that suited them. And I agree, collectors may not have notated in actual key sung,

Does it matter if it is changed? surely not!
Does changing the Key also change the song? no, I don't think so....

...that said, I used to have a theory while spending time as a student busking on solo fiddle that people in the street gave more money for tunes played in G major than for D major. No idea why - I speculated that culturally maybe people were predisposed to find G major happier. Although it could have been to do with pitch - maybe higher pitched or lower pitched worked better to cut through busy street noise. I never did invest the time to do a proper scientific experiment (I was studying maths & statistics at then time so was thinking about it in quite a geeky way) but I'd have been interested to try it, with the same tune in different keys, to see if in the long term it really did work out that some keys were better money makers. Of course, I would have had to consider other factors like geographical area etc as well otherwise I wouldn't have really been able to generalise my results outside of the towns I normally frequented. But, alas, I never did collect the data and am far too busy these days to spend the time doing the busking.