What must I recreate before moving a domain between Cloudflare accounts to avoid DNS or SSL downtime?

Treat it like a new Cloudflare zone, not a move that carries everything across. Before you switch nameservers, export and validate the old DNS records, import them into the new account, and manually recreate the Cloudflare settings, add-ons, and subscriptions your site relies on. Remove DNSSEC first. Also get TLS ready: Cloudflare says certificates from the old account don't transfer, so reissue them; custom certificates can be uploaded to the pending zone ahead of activation.

Then add the domain to the target account, change the nameservers at the registrar, and wait for the new zone to become Active. While it's Pending, Cloudflare won't proxy traffic and visitors can reach the origin directly, so make sure the origin DNS, firewall, and certificate are safe for that window. If Cloudflare is also your registrar, use its separate inter-account registration-transfer process after the target zone is set up.