> '''
> -c, --current-version
>
> By default, convert-ly updates the \version string to the lowest
> necessary version. If this option is given, the current LilyPond
> version (2.25.27) is used instead.
> '''
>
> So, IIUC, by default convert-ly should update the \version string only
> if the file contains a syntax change which requires the version bump.
> The -c option instead would update the version number regardless of
> any syntax change.
>
> Well, this is not what I'm experiencing.
> By default convert-ly is updating all the \version strings even if
> there's no syntax change. [...]
>
> Am I missing something?
Yes, you do :-)
-d, --diff-version-update
only update \version number if file is modified
Werner