Once you have associated an issue type to a workflow in a scheme, you cannot add a new association for the same issue type. There is also a no edit option to change the association. What you need to do is to delete the existing association and create a new one using the following steps:
- Browse to the workflow scheme's details page for the workflow scheme you want to configure
- Click on the Remove link for the association you want to remove
Once an association is deleted, you will be able to create a new one for the issue type. If you do not assign a new workflow to the issue type, the workflow with the All Unassigned Issue Types option will be applied.