To be perhaps slightly more accurate, it's like a major version 5.

Almost (if not) all missions include changes of some kind; the manufacturing blocks each have significant, incompatible differences in parts.

Upgrading the grid fins would be a minor version change, modifying the engine to increase its throttle depth a major one. I don't know, but it seems like there would need to be significant re-tooling for each new block, and many parts would be incompatible with previous blocks.