Sketch Library updates silently dropping symbol overrides — which structures survive?

Symbol overrides after Library updates

Sketch Libraries let a document reference shared symbols and apply local overrides such as text, fill color, and size. When the library is updated to a new version and the document runs Update Libraries, those overrides are sometimes preserved and sometimes revert to the library definition. The behavior appears to depend on whether the updated symbol's internal structure still matches the overridden instance, but this matching rule is not clearly documented.

The goal is to define a safe workflow for teams that rely heavily on overrides — including nested symbols — so that updating a library never silently discards local changes. Assume a recent Sketch version (71+), since behavior has reportedly differed between releases, and treat any version-specific claim as needing verification.

Constraints: detached instances are out of scope, and duplicating every override into a local symbol defeats the purpose of a shared library.

1. What structural changes to a library symbol (layer renames, added layers, nested symbol swaps) cause existing overrides to be dropped on update?

2. Is override preservation deterministic per Sketch version, and how can it be verified before rolling an update out to a team?

3. Is there a supported way to preview or roll back an update that would reset overrides?