This reply is for certain way too late for the person who posted the question, but I stumbled across this post when I was searching for how to do something similar. This is how I ended up doing it:
- Right-click the polygon layer and select Symbology
- In the Symbology pane under the Primary Symbology tab, select the Scales tab.
- Click "Add alternative symbol" for each polygon symbol that you want to show a single symbol for at larger scales.
- Adjust the Scale Range so that the original symbol and the alternative symbol are shown for different scales. At this stage the two symbols are the same, but this will be changed in the next steps.
- Click on the Symbol for the alternative symbol that you want to display as a point symbol at larger scale. (If you hover the mouse over the Symbol the text "Format symbol" will appear
- In the Format Polygon Symbol pane that will appear after the click in the previous step, select the properties tab and then click on the Structure tab.
- Click on "Add symbol layer" and select "Marker layer" from the dropdown.
- Delete the other layers below the newly added symbol layer as these will no longer be used.
- Switch to the Layers tab and select the Shape marker layer.
- Format the marker as you would like to display it by e.g. changing its Form, Color and Size.
- Under the Marker Placement section, change the Placement to "At center". This is important to do to only show one marker symbol at the center of the polygon.
- Apply the changes and you should be all set with a point symbol for larger scale and a polygon shape for smaller scales!
Also see these document pages for some related information:
https://pro.arcgis.com/en/pro-app/latest/help/mapping/layer-properties/scale-based-symbol-classes.ht...
https://pro.arcgis.com/en/pro-app/latest/help/mapping/layer-properties/position-and-place-marker-sym...