No switch back.
The extension works. It allows me to change the user agent, but for the whole browser. Not just the current tab.
On Teams, the messages suddenly got all scrambled. I had a mix of old messages in random order. Every refresh was giving a different set of old message. It's likely a bug at Microsoft, but still, that's not usable.
There is no way to revert back to the default user agent. I set it to my browser and OS, but Teams stop working saying my browser was not supported. I had to uninstall the app to continue using Teams.