Hi,
Could you please share what is your exact business scenario? when and in which event you are triggering the plugin? and when you want to fire the onchagne method?
Onchange event trigger in client side but plugin execute server side , onchange method wont be trigger if you update fields using plugin. For that you may need to execute some client side code and retrieve the data using API to check whether data has changes or not and based on that trigger onchange method. In addition my personal suggestion is to write all the logic in the plugin code if there is no dependency at client. You can also now trigger business rules with scope entity if your requirement matches with Business Rules.