A plan change never deletes anything immediately. Your account and every project you own stay exactly as they are, with full read and edit access, the moment a plan change happens, 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.
The lifetime window every Free project has
Every project on the Free plan has a lifetime window, see "How long do Free plan projects last?" While you're on Plus or Creator, every project on your account is exempt from it. Cancelling doesn't end that exemption right away, you keep it for the rest of your paid period, but once that period actually ends, each of your projects gets its own countdown, starting from that moment (or resuming, if it still had time left when you first upgraded), and you'll always get advance notice by email before either step (freezing, then deletion) happens.
If you now own more projects than your new plan allows
Every project counts toward your plan's limit, including a frozen one, not just ones you're actively working in, see "Free, Plus, and Creator plans compared." If a downgrade (or a "Refund Now" refund) leaves you over your new plan's limit, for example 5 projects on the Free plan's limit of 2, every one of your projects freezes immediately, including their published pages, and you're asked to choose which ones (up to your new limit) to keep the next time you visit your Dashboard. You can't create a new project until you've chosen.
The ones you choose get a fresh lifetime window on the Free plan, exactly as if newly created. The ones you don't choose are not deleted immediately either, but they're not exempt from deletion the way an actively kept project temporarily is, they follow the same schedule any other expired Free project would, and will eventually be permanently deleted unless you upgrade again first (which unfreezes everything on your account immediately, no choice needed) or delete them yourself sooner. You'll get an email the moment this happens, with a direct link to make your choice.
Beyond project count and the lifetime window, we will never silently delete a project, remove content, or lock you out of the editor just because of a plan change. Deleting a project yourself works exactly the same on every plan, see "Deleting a project."
Tip: Planning to downgrade with more projects than the new plan's limit allows? Delete the ones you don't need beforehand if you already know which you'd choose, that way there's no choice screen waiting for you afterward.