it depends on the size and complexity of your application. If it is very small it would not be hard, but if it is small\medium\big and complex I would say the upgrade will be the same as you would rewrite the app from scratch (but copy-paste some logic).
Note, Ember 2.x does not have views, partial views, so you will need to re-implement them with components. You will need to use ember-cli with the structure of files it proposes. By default, ember-cli expects you to use ES6, npm, so you need to know how it works. If you use Ember-data, it would need your backend to support json api specifications http://jsonapi.org (by default, ember-data 2.x uses JsonApiAdapter, but ember-data 1.x used RestAdapter)
I have couple applications written on Ember 1.9. When I need to upgrade them I would just look at the task as I need to write the apps from scratch