Technically, you don't gain the 'expert' title at 6,000 points. You gain the editing powers, but we have to manually assign you the title. These days, we're inclined to promote experts a bit earlier than 6,000 points as well.
If you have the expert title, then you will keep editing powers no matter how low your points go, unless we manually take the title off you. However, if you don't have the title and get downvoted back below 6,000, then yes you'll lose editing powers. However, there's really no longer a way that will occur.
The other roles aren't points-based at all, so there's less to talk about there.
