I built Sigil and… it just works!
I saw in the commit that if a version isn’t specified, we assume it’s version 1.0.0.
I think the new plugin will add the version when saving, even if it loads a file without one.
And honestly – I don’t see any bugs. It just works, maintaining backward compatibility, though with that one small difference I pointed out in my comment: now, even with the default character layout, subsequent lines fill the entire width of the window.
Of course, I recommend that other testers conduct additional testing.