You don't call Envoy to make changes. You either update the config file, or you implement an API server with a specific set of endpoints, then have it serve the updated values back. It's a bit tricky, but entirely doable. https://envoyproxy.github.io/envoy/intro/arch_overview/dynam...

(You can do this in nginx, too, and it's similarly annoying. This is a big part of what we do at Turbine Labs -- a UI with a slider between blue and green, and a place to host all those stats, so you don't have to manage it!)