Each edition incorporates what's been learned since the last — dated, versioned, and visible, with a changelog you can read. The configuration you build in Unit 2 survives minor revisions; breaking changes carry a migration the kit performs, not you — and the portable shop it compiles simply recompiles from your settlement, never hand-patched.
Buying once includes every quarterly edition, every vendor-evaluation refresh, every agent and lesson update that ships under the 1.x line. The intent is a living body of work you can return to as conditions shift while you are using it.