I think I understand what Allan was getting annoyed about.
If someone picks up a song of mine, and sings it when I'm not there, or even when I *am* there - they can change it about a bit, move the chorus around, even tweak words a bit, and I'll still be flattered that they liked the song enough to perform it.

However, what Allan was describing is someone who was invited to *accompany* him, and took that as permission to re-work the whole song.

That's a completely different thing, and I'd find it challenging to respond politely.

I've also found there are people who cast themselves in the role of "critic", and submit unasked-for and unsought opinions on structure, lyrics, melody, etc etc. Some of these critics do not take kindly to any observations on their own work. I guess that's what sighing is for.