SOA and Versioning

Funny, I thought that 'BigCommerce' was a fake name like "Acme Corp" or "Foo Co".

Versioning APIs is hard. Anyone have any good reading / experience to share? How do you prevent too much code duplication across folders? How do you expediently insure that code that is shared doesn't break a distant dependency? Should you create lots of new versions or only for large changes? Does something like Semver help? I don't feel like I've seen a happy solution for this yet in my work and would like to learn more...