Once a record is associated with an application, it cannot be changed. You will need to recreate it (and related records) in the global scope.
I don't make the recommendation lightly as it can have disastrous effects. You may want to consider the manual recreation approach. This can be done a bit quicker by exporting the record (and related list records) to XML, then CAREFULLY editing the XML to replace the application name and sys_id replaced by global, then removing the original records and importing the updated XML file.
If you choose this method, I highly recommend testing the import on a separate instance (e.g. get a free personal developer instance from developer.servicenow.com) before removing the old one.
