No, it just overwrites. If I try to run the same script in Python 3.6.8, it stops the locator on the server and then fails to upload the service definition.
And I've found another issue. I create the locator in Pro/Python 3 using arcpy.geocoding.CreateLocator. This tool has no capabilities parameters (geocode, suggest, etc). I can't use the older tool arcpy.CreateAddressLocator_geocoding because suggestions are totally screwed up when the service is consumed in a web app. If I publish the locator straight from Pro (created with arcpy.geocoding.CreateLocator), everything works. If I create the locator in Python 3.6, and then publish it in 2.7, then suggestions are turned off. Turning them on and restarting the service has no effect. So no matter what, I have to physically publish the locator to the server or it won't work correctly. I need this process to be fully automated end to end, because the address points data I'm using is updated by the city on a bi-weekly basis.