But I'm curious to know why SoP is unwilling to change words when he seems quite at ease with the notion of changing or importing tunes. What's the difference?

I suppose it's because I'm an essential creative / improvising / mediumistic / intuitive musician, and not a song writer. Also, many ballads I sing I come across in written sources, which makes me think about them differently than if I heard them sang. Thus, whilst I never alter words, I will quite frequently do the same ballad in various different ways, such as Child #79 - which I might sing like THIS (i.e. in a traditional style even though the melody is my own) or as I do in Tack #10 HERE (i.e. freely intoning the libretto as part of an entirely improvised performance).

To tie in with my previous post, such musical creativity I see as essentially mediumistic to an inner creative process, defined by the hard-wiring of my brain and the cultural software I've been uploading these past 48 years. My idiosyncrasy is, therefore, the consequence of my cultural / collective / communal human context.