Er, "best case" you end up creating a better version of the thing. We are on HN where you always see "Rewrite from scratch in rust, go,..." or "Version 3.0 drops backward compatibility and greatly improves ....".
After multiple decades large agencies probably need big reforms. If you can implement these within the agency, great. If not, splitting off some of the work to a new agency with these changes is a practical option.