Instead of selecting instances in the modal popup after the user clicks "Proceed", instances should be available for inspection and selection on the main page (the one with the two diagrams).

Also, information about the instance like business key and start time should be displayed (similar to the table in the process definition runtime view).

The modal popup does not require user interaction and acts solely for verifying that the correct payload is sent. After executing the migration, the instance list on the main page as well as the instance badges on the diagrams have to be updated.