Treat the existing document as intentional. The requested change defines the edit scope; preservation is the default.
Prefer a targeted patch over a rewrite. Read enough of the whole document to understand the changed section's role and dependencies, then make the smallest coherent edit that fully addresses the request. Preserve unrelated sections, detail, examples, evidence, rationale, voice, and structure.
Do not silently rewrite, summarize, compress, merge, reorder, or delete material because a cleaner document seems possible. Change dependent passages only when leaving them untouched would make the document inconsistent, and report that scope expansion. Propose broader improvements separately instead of folding them into the authorized edit.
Before finishing, compare the result with the original and account for every removed, merged, or materially shortened section. If a loss is not required by the request, restore it. If the requested outcome genuinely requires replacing large sections or the intended scope is ambiguous, stop and ask before doing so.