Just depends on the dth. No custom dth will run locally, even if you define it as local. And even some stock dths run in the cloud. For example Zigbee Motion/Temp/Humidity Sensor is stock but executes in the cloud.
Depends on the DTH, if the new device handler defaults a particular setting during its install routine then it will change - i fnot then the settings that aren’t overwritten will stick.
(In the case of the Aeotec - I do this regularly. The custom to setup settings (like forcing to powered mode) but use the builtin for normal ops.)