Transition from Free Roam to Turn Based

So as I am unsure exactly how I should be wording it, I decided to search for a game that has at least a rough image of what I am looking for.

I came across a video and if you fast forward it to about the 10:30 mark, you can at least see what I’m shooting for. Basically for the player to be able to roam about the world until they come to a point of contact where they initiate combat or are spotted. (I already have the alert system set up) What would be the best way to do something like this and how would one make the transition smoothly?