A plan change never deletes anything automatically. Your account and every project you own stay exactly as they are, with full read and edit access, whether you're on Free, Plus, or Creator.
What actually changes
What changes is which benefits currently apply to your account. The clearest example: a custom domain connected under Plus or Creator needs that plan active again to keep resolving, if you drop to Free, the domain connection itself isn't deleted, but it stops being served until you upgrade again.
What never happens automatically
We will never silently delete a project, remove content, or lock you out of the editor because of a plan change. The only way a project is ever deleted is if you explicitly delete it yourself, see "Deleting a project."
Tip: Planning to downgrade with more projects than the new plan's limit allows? Nothing forces you to delete the extras — you keep them, you just wouldn't be able to create new ones beyond your plan's limit until you're back under it (by upgrading again, or deleting some).