SDK 6.x:
It is possible to change the license key at runtime by creating a new data capture context instance, which is initialised with the new key. Note that you also need to create a new data capture mode and data capture view for the newly created context to be able to use it. You do not need to close the application as the whole operation can be done at runtime.
SDK 5.x and older:
It is not possible to change the license key at runtime. It is set when the first barcode picker is instantiated and it will be used for as long as the application is running. To change the license key, you have to completely close the application. It is not sufficient to put it in the background, e.g. by going back to the home screen.