How can I make customised constraints?

If I start the call with mediaConstraint = {audio: true, video: false}

How can I later turn the video: true and let the other peers know I have started video too? This I need because I have feature where user can start with audio only and later while having the call he can turn on/off the video. And if I enable the video constraint from the beginning it will ask for camera permission on the app which is bad user experience. How can I handle or fix this issue?