Changing a Selenium Proxy Means Planning for a New Browser Session
Treat WebDriver proxy settings as session configuration and account for state when changing browser routes.
Standard WebDriver proxy settings are supplied when a browser session is created. There is no general WebDriver command that edits those capabilities midway through the session.
For portable automation, create the next session with its intended proxy configuration and close the old session when it is no longer needed. For permitted public-browsing tests, choose a browser-compatible endpoint from the free proxy list before creating that next session. Browser-specific extensions or debugging interfaces can offer other mechanisms, but they have separate support and permission requirements.
The browser session carries more than a proxy
A replacement session changes the context in which the test runs. Cookies, storage, authentication, and open pages need deliberate handling. A clean session may be exactly what an isolation test needs; a stateful workflow may need a supported way to establish its application state again.
A changed exit address also does not reset the destination account. If the workflow signs back in to the same account, the service can relate those actions through that account regardless of the browser's proxy.
Keep these choices in the test plan: which steps require one stable route, where a new browser starts, and what application state it should receive. That makes failures easier to interpret than changing an extension's global setting while existing connections and requests remain active.
Finally, inspect the actual remote browser's route. A proxy setting on the machine running the test script can affect its connection to WebDriver without configuring the browser on another machine.