When the app is deleted, all locally-stored data in the app will be deleted. A user will no longer be able to share the stored logs should they be contacted by a contact tracer. They need to re-install and set up the app again to participate in TraceTogether.
If you've changed your phone and retained your original mobile number, no worries. You can download TraceTogether on your new phone and register with your mobile number. The TraceTogether data in your previous phone will not be ported over to your new phone. It will stay in your previous phone for 25 days, after which it'll be deleted.