Will my third-party extensions / custom modules break?

Some will. The pre-upgrade audit (free with every booking) catalogues every extension + custom module on your store and flags incompatibilities before we touch production. For incompatible extensions we either: (1) upgrade to a 2.4.9-compatible version from the vendor, (2) patch the extension ourselves if the vendor is slow, or (3) rebuild the functionality natively. You get a full report before any work starts, no surprises.