@Mtiede Why not try it? The worst that would happen is the call be dropped and a new video call be started.
If you are using the carrier supplied video calling, I would think it would just be transparent. But generally carrier supplied video calling only works if you and the person you called are both on the same carrier.
If you are using Duo as your video calling app but started the call from phone, then the call will probably be dropped and a new video one started in Duo.
I was kind of looking for HOW to do that. So I can't "try it". I was looking for someone that has an already tried and true mechanism that could be shared here. I don't video much at all and usually would skype someone although I have also got Duo installed.