Natasha, perhaps I can add a thought which might be useful in terms of setting some context for the concept of alterations to traditional songs (something I've done and do a lot - I'm currently writing a whole musical made entirely of trad songs re-written, in fact).
There is a theory which views the tradition as an essentially destructive system, albeit one heavily influenced by periodic acts of reconstruction.
We use the analogy of a river, and of course they do more than flow - they also erode.
People mishear and forget words, flatten out tunes, muddle one song up with another - and generally degrade the original writer's intention - and you can see this clearly when you listen to source recordings, or in fact visit a club and listen to people singing songs you know.
I think most people who ferret about in the archives looking for forgotten songs to present to new audiences today will find (as I'm doing right now as it happens) that a certain amount of editing, restoration, reconstruction, reinvention etc is necessary if the song going to stand up against the rest of the repertoire - and I personally think that there have always been people who did this rebuilding work (it may be one of the main causes of variants in fact).
Some people were and are happy to present a song exactly as found/heard - either because its condition (even if parlous) is part of the story, or because they're just not comfortable with changing things - but most of us will 'correct' any obvious 'errors' that seem to have crept in.
I know you're interested in deliberate alteration for political (small 'p') reasons, but it's worth bearing in mind that the requirement for alteration often comes first - just as one would need to replace the missing leg on an old table and give it a polish before you could sell it as an antique.
And of course when making repairs to a song, one effectively takes on at least the mantle of a songwriter - and in so doing are liable to introduce your own personal value system into the work, possible unconsciously.
Tom (in Leeds, as it happens)