My first posting was looking at it just for trad stuff with no living songwriter. However, I think I'd still go for the same answer even where the writer is still alive - but only for casual singers like myself - it would be a bit cheeky to change anything if you were singing more formally (I s'pose I mean where people would look to you as an authority sort of).
If a 'proper' singer wanted to change anything then it would be reasonable to approach the writer & get their permission?
I expect different writers would have different attitudes to this question. Any writers care to comment???

Kris