Replace a page (rebuild it, often at a new URL) when its structure, angle, or intent is fundamentally wrong for what now ranks, and update it when the foundation is right and only the content has aged. The line between the two is one question: does the page’s underlying premise still fit the search results, or is it built for a query that no longer exists in the form the page assumed?

Updating works when the bones are good. If the page targets the right intent, is organized in a way that matches how the results are now structured, and just contains stale facts, thin sections, or dated examples, then refreshing the content fixes it. You are improving a structure that is still sound, and you keep whatever ranking equity the page has accumulated.

Replacement is the answer when the bones are wrong. If the search results have shifted to a different intent (the query that used to want a definition now wants a comparison, or what was an informational page now sits among commercial results), or the page is structured around an angle the current top results have abandoned, no amount of content editing rescues it. You would be polishing a frame that was built for a different building. In that case a fresh page (sometimes on a new URL, with the old one redirected) gives you a structure designed for what the query actually wants now.

The trap on both sides is reflex. “Always update, never replace” keeps you patching pages whose entire premise has expired. Replacing on a whim throws away ranking equity and stability for pages that only needed a refresh. The foundation-fit test cuts between them without either reflex deciding for you.

Before you choose, look at the current top results for the page’s target query and ask whether your page is the same kind of thing, built around the same intent and structure. If it is, update it. If the results have moved to a different kind of page entirely, stop patching and rebuild for what the query has become.