Just came here to post that very same request 
I am endlessly confused by why some options can or cannot be set depending on whether:
- you’re creating a draft (can choose blog but cannot choose date and cannot choose slug)
- you’re editing a draft (can’t choose blog, cannot choose date and cannot choose slug)
- you’re editing a published article (can edit everything)
This makes basic tasks needlessly confusing and cumbersome
Related discussion: Publishing a draft post, workflow