If I edit a Sponsorship Box in the Outpost Control Center, does it change posts that already have the Sponsor Box in it?
No. This is a static item. You can edit a Sponsorship Box in your library, but any changes you make only affect future posts you put that box in.
How do I remove a Sponsorship Box from an existing post?
Deleting an existing sponsorship box in a published post is simple. Find the post in the Ghost editor, click on the sponsor box and then delete it. Save your changes and the post will be gone.
What happens to Sponsorship Boxes if I stop using Outpost?
If you stop using Outpost, the existing Sponsorship Boxes in your posts will continue to show up. You will lose access to the library and the tool for making new ones, but the existing ones will not be affected and will continue to show up in past posts.