How would you plan a safe Angular major-version upgrade?

For a large application I canary the new build, observe error and performance deltas by release, and retain a fast rollback path. I avoid permanently using broad legacy compatibility flags because they conceal the remaining migration cost. Codemods are followed by semantic review: a syntactically valid change can still alter provider scope, rendering timing, or test behavior. The completion criteria include supported dependencies, clean production telemetry, updated runbooks, and no unexplained migration residue.