I'm really less concerned about larger projects like Apache which have a clear published versioning strategy because they're also the kind of projects that are less likely to bite you when doing what should be safe updates within minor / bugfix versions.
There are some good standards out there for versioning. But honestly the best thing you can do as a developer or sysadmin is to treat every version update like a breaking change and run it through your dev and test environments before pushing to prod.
https://www.folklore.org/StoryView.py?project=Macintosh&stor...
With my tongue firmly still in my cheek, I'd say increment the version number by the difference. So if you deduct 5% of the code and add another 15%, then increment the version number by 20%.
(To be clear: I disagree with the suggestion, I just don't think this is a strong argument against it.)