Upgrading a Lumos site

Two things have to be true when this finishes, and they pull against each other:

1. Nothing the user wrote is lost. Their overrides, their pages, their tokens, their components.
2. Nothing is left on old conventions. Renamed props renamed, moved components moved, retired patterns retired.

Keeping their work by leaving it alone fails the second. Modernising by overwriting fails the first. The way through is a three-way merge — what they started from, what they have now, what ships today — and the discipline to ask rather than guess when those three disagree.

Where the site started

An upgrade is only as good as its knowledge of the starting point.