Very late response
You can use pgAdmin to log into the Postgres database (use port 4432 instead of the default 5432) and manually update the Stream ID for an app. Be sure to stop the Qlik Sense Repository Service while you do this.
UPDATE public."Apps" SET "Stream_ID" = '8c661fd2-5105-499f-8458-689370731a37' WHERE "ID" = '847ef781-2ede-46ea-b129-764cbabc8f01';
(Stream_ID is the ID of the stream you want to move to, ID is the ID of the app that you want to move)