Two things come to mind - one is that a lot of traditional Scottish tunes are often played with one note "off" when using bagpipes (aka the celic wind banjo), because there notes on the fiddle which do not exist on the pipes. The other is those arty types who try to muck about with folk tunes, taking them out of the mode and into a standard classical scale to suit their tender ears and tastes. It upsets them, for example, to find that a song apparently written in Em should have two sharps in the key signaure ( because it's second mode of D) rather than the one sharp of Em. This is not so bad until they change the key signature to Em and leave out the accidentals. The number of bad versions of Greensleeves is inestimable - and that's just one example. Changes are acceptable - do it if you can't reach a note, or if you have something new to offer (I changed the timing on the chorus of Mad Tom of Bedlam 20 years ago and everyone I hear sings it that way now). If you aren't sure, try out both versions on people that should know - get 'em in a corner and ask! I've got some good advice that way, and even people that are famous for not suffering fools gladly can be very free with their advice if you ask them nicely and don't take all day to do it.