Do i have to pay extra for updating a already existing app (in classic) to "lightning ready" apps ?
While updating a app to "lightning ready" - will my existing data in the app will be removed or can i get the same data in lightning?
Ad. 1. If you bought app you have to wait for the update from original developer. Be warned - not all apps are being updated, best to ask app creators if they plan to release update for Lightning.
Ad. 2. Under Lightning there's still old SF database so no need to do any data rework, all data will stay intact.