In blender 2.79 there was an ability "to circle", i believe, that took a series of vertices and made a circle with them. i am not finding that ability anywhere in 2.8, is it gone?
Let's say you have a plane like the one in the picture below, to transform it into a circle you want to use the Subdivision Surface Modifier.
To do this, select first your model, then select the little wrench icon you find on the left of the properties panel which is on the bottom right of the user interface (the one pointed by the red arrow in the second picture) then click on Add Modifier. You're going to see a dropdown list, select the Subdivision Surface Modifier from there.
Once you've selected it you're going to see something like in the second picture on your properties panel, while the plane will look like the picture below.
What you want to do now is increase the value of the View and Render properties on the modifier. These basically increase the number of subdivisions on your model: the first one is just for the viewport, the second is for the actual render of your model, if they are different your model will look different in the viewport and when you render it. By setting both values to 3 I was able to transform the plane into the circle in the picture below, which I think is enough.
You have to activate an addon called loop tools.