Will the HANDYPARKEN app still work? Do I need to replace it with the EasyPark one?

The HANDYPARKEN app will work in your city until it is inactivated. Then you will no longer be able to start a parking session in the HANDYPARKEN app in the majority of cities in Austria, except for the city of Vienna, where HANDYPARKEN will remain available. Using EasyPark instead means you will be able to park in nearly all of Austria.
You will still be able to access your parking and payment history in the HandyParken app or website.